Problems with Marth
Alright first off, I doubt everyone will agree with all of this, and I'm not trying to say these problems are necessarily unique to Marth, but all in all I think they add up to create a pretty serious up hill battle for him in trying to solo win a major tournament.
Anyways, this isn't really anything new. After every major that Marth doesn't do particularly well at there's always an outcry about Marth being bad or overrated or whatever, but while I think Marth has particular flaws that really hurt him, the bigger issue in Melee imo is that there has been a pretty significant paradigm shift over the last couple of years in what characters and strategies are truly viable for winning major tournaments.
First of all, **** the tier list. Listing characters linearly is very misleading because it indicates that all of the characters are playing the same game by the same rules and that just isn't necessarily the case. The fact that Jiggs and Sheik can be ranked side by side and that anyone can actually assess one being a smidge better than the other should indicate that it's completely bogus because one character is highly variable, one of the most if not THE most abstract, difficult characters to measure in the game (Jiggs, obviously). The other, sheik, is pretty much the most conventional straight forward character. You don't have to dig very deep to understand why sheik is good, her entire design is clearly built for efficient damage racking and killing - winning, if you will.
However, sheik still plays within the boundaries of the game. She doesn't really have the proper tools to camp MOST characters to time out, nor does she have overwhelmingly good offensive tools for getting the hits she needs to perform her great punishes. She is the best character of these types of characters, who have to find some sort of middle ground between offense and defense in order to compete, not gifted enough at one extreme or the other.
These characters are every character that isn't Jiggs, Peach, Fox or Falco. In addition, all of these other characters (for the most part...IC's are another outlier character that I don't quite know what to make of yet) are the characters Sheik particularly tends to thrive on beating, and it artificially bulks her up on the tier list because she is quite out matched against the top 4 characters. I don't think Sheik can win a national either.
What does this have to do with Marth? Well Marth is very similar to Sheik I think, though sort of directly inferior in a lot of ways, and better in a couple of ways. Marth's mobility and having guaranteed grab combos on Fox, for example, makes him better equipped for beating Fox, but in the vast majority of matchups, sheik is the one with the guaranteed grab combos and her lack of mobility is not as apparent against non-Fox characters. Sheik is also oddly not that good at corner pressuring imo, as if you are standing near the ledge under a side platform and sheik stands in the middle of the stage, she isn't actually that threatening because her extended range consists of running at you with either a dash attack or grab, or just standing needles. With no platform to do tricky stuff on or use needles practically she is kind of naked and it seems like a bit of an odd flaw but for whatever reason if you don't play scared with your back to the ledge you might find sheik to be not that scary. Marth's stage control isn't much more practical, though...
Stage Control
A lot of people say Marth has great stage command and controls space amazingly but really, he's quite mediocre at it in comparison to what he's up against. The formula for great stage control is to completely own the space you occupy and then to extend pressure into the space in front of you in some way. So for Falco, dair/nair/shine/utilt all very very strong close range moves that require literally minimum frame advantage to land and to top it off, all of these easily lead into big damage and potentially a stock. Very threatening. With that established, he then uses lasers to pressure forward and simultaneously harrass whatever advantageous spacing you might be trying to set up against him. Jiggs and Peach have similarly amazing tools for this. Jiggs particularly is basically camping you very severely initially to stuff any type of offensive approach you might try and once he gets you to start camping her back then he starts inching forward with those bairs and taking stage away from you until he has cornered you, which ends very poorly for the opponent.
But at least Falco will be very vulnerable if he actually gets hit, where as Peach and Puff live forever and are generally not very afraid of being off stage, therefore making their camping absolutely legitimate as a strategy. This also affords them to take more risk (if they want), because the risk-reward in spacing wars is constantly skewed in their favor. Peach would love to trade with Marth all day long. The pressure is on Marth to space perfectly, not her, as if he doesn't he will get the poor end of the trade and also it will be more costly for him as he is severely vulnerable whenever he is forced out of position or off stage. He also sucks at close range, and I mean absolutely sucks. His sword is completely CCable at that range and his jab is horribly slow for practical jab use. He basically has grab, which any character has to begin with. For example, rolling into Marth can actually legitimately **** him if you let it, so this is something you have to guard against actively in order to not get *****, where as the good characters punish rolls pretty freely on reaction with fast moves that just dominate at close range. Another damning aspect of his stage control is that he just...doesn't have much control. He doesn't have any tools that force approaches like a competent projectile, but he doesn't have great offensive tools for pushing the pace either. He's constantly forced to space around what his opponent is doing, which sucks because spacing is his only solid universal advantage but when he's forced to space defensively constantly he's vulnerable to every character's tricks for forcing misspacing (and every character has them and competent players will execute them). Marth is also lacking in CC options defensively. CC grabs are amazing if you can get them but while his grab range is great for a grab, it's still not as ranged as, say, Fox's dtilt, or some similarly quick move that pretty much all other competitive characters have to defend themselves adequately in that situation. Even with Marth at 0%, Fox's back air can still outspace a Marth cc grab if done properly.
Glass Cannon, but where's the Cannon?
I want to focus on the "Glass" part first. I mean, spacies will always argue that the balance to their character is how harshly they are punished, and they are punished quite harshly, but Marth is right there in line behind them man. I think it is really common for people to overlook just how completely boned this character is off stage. Absolutely among the easiest to edgeguard and easiest to gimp in the game. It isn't just the direct combos that are so punishing - they're pretty bad but not the worst - but the sheer fact that he sucks at combo breaking, sucks at close range, sucks when lofted above people trying to get back on the ground. He becomes a complete punching bag in these spots. If you give spacies any daylight in a combo they will shine you and completely flip the momentum. Peach will dsmash you. Instant momentum shifts are amazing. Marth is just hoping you will let him escape so that he can try to start all. over. again. Nothing comes cheap for him. He just wants you to give him an opportunity...to have another opportunity. This is really a massive disadvantage. He's also really screwed coming off the ledge. No actionable invincibility really hurts a character that is so vulnerable offstage. I mean yea, Peach doesn't have that either but Peach lives double what Marth does on the regular (and also has semi viable on-stage recovering options, where as Marth recovering to the stage is basically conceding getting hit again and only does it when forced to). Tons of characters exploit the crap out of Marth getting off the ledge, and against someone like Armada just forget it, he's not letting you back up.
And yea, where's the cannon? His punish game is extremely fragmented in comparison to any character going down the list to even mid tiers. Spacies, I got it, he's competent there, but everywhere else his punish game is consisting of hard read after hard read. I don't think some people really properly value guaranteed-ness in melee, as when things are not guaranteed in melee you instantly jump from 0 to somewhere between 4 and 8 options in most scenarios. In the long run this leads to massive inconsistency in his punish game. You will be fishing for kills, at times, in absolutely all matchups while your opponents kill you methodically and routinely stock after stock. In some matchups like Peach/Puff/Samus you KNOW going in you will be fishing for kills basically every stock. I understand that all characters at times have to fish for kills, but the better characters can generally fish with something pretty safe (though nothing is completely safe), like spacies' bair. Marth finishers like utilt and fsmash are extremely laggy and punishable in comparison, making him very vulnerable when trying to get kills. I know every Marth has been in that situation where they start off a match losing the first stock despite doing a lot of damage to the opponent, then coming back and fishing for that kill only to get opened up on the next stock before that kill actually comes. Worst feeling in the ****ing world.
And don't get me started on playing from behind with Marth. If you're down 2 stocks to peach or jiggs you can go ahead and unplug your controller. He's not built for comebacks. He's extremely vulnerable to conservative attrition-based play, which lots of characters can launch right into very effectively once they get leads on Marth. Spacies, on the contrary, are the offensively gifted characters in the game and while they can be played extremely gay and campy if they themselves have leads, if they happen to fall behind they can dig in their bag of tech skill and find ways to bring it back. This ultimately is what makes spacies the best characters imo, as they are capable of running away and even timing out a ton of characters if they really wanted to, while simultaneously being capable of actually approaching and **** stocking for come backs if they are necessary. Most spacie players play spacies because they want to be able to play offensively, though, so time out spacies have not yet become much of an issue (though if there were more money in the game and more majors, sponsorship, etc I think it would definitely become pretty common).
Outgunned by Spacies
I think right now as of this moment Marth has very decent matchups with Fox and Falco (though Falco is much much more difficult if they know what they're doing). Going forward, though, I don't see much upside or innovation really for Marth where as the rate at which Fox tech skill is starting to take off, I think it's very likely that he will eventually fall behind against them. I don't think either of them can truly camp Marth to death but I think offensively they will just overwhelm him, exploit him once out of position, and destroy him off stage. As they get better control over their shield pressuring tools Marth will lose more and more of those pressure exchanges, as well as spacies that take advantage of ALL recovery options available will live longer, be gimped less, and Marth will just not be able to keep up. Abusing ledge invincibility severely hurts Marth's ability to pin the spacies down as they get off the ledge. There is nothing you can do about this but concede space and continue to play defensively. Sorta sums up Marth in the metagame, constantly giving up ground. Marth innovation: Space....harder?
Lack of Pressure
The top characters are all quite good at shield pressure in their own ways and manage to be pretty safe and destructive in doing so. Sheik also has pretty good pressure, though still not nearly as safe or effective as the better characters'. Good shield pressure allows you to forgo things like whiffing grabs to well timed sidestepping, which Marth has to deal with constantly (Vudujin told me at Apex: "Marth loses to sidestepping") and this is one of the major reasons that bad characters can still give him tons of problems. If you have a fox nair that lasts long time and has a shine following it up you're not even going to bother with sidestepping (making getting grabs even easier!) but the lack of actual safe shield pressure for marth throws off the balance of things here. Most of Marth's "pressure" ends up coming from standing and waiting, hoping you will get scared and sidestep early. This works sometimes, especially on people who suck against Marth, and don't realize that while he stands there doing nothing you could go right into your own offensive pressure (just ****ing hit him). Marth has to wait for sidesteps cause he has no other really strong ways to punish it. The real point is, while it's cool that Marth can just stand there and scare some people into doing things like sidestepping or rolling for no apparent reason you have to actually hit their shield to ensure that what you're doing is safe and then also be able to punish the options you have now limited them to by forcing them in shield. Better players all understand this and will often go on the offensive if you get caught staring at their shield for too long. The best type of Marth shield pressure I've been able to come up with was cross up nairing past someone's shield. This will cover a sidestep and also land you relatively safely behind them which you can then continue pressure with a side B or turn around grab or dash away hoping to bait something out from them. It works okay but it takes more conditioning than traditional shield pressuring tools do, and, like standing and waiting, you can get called out for it and attacked early before you get it off.
Ratio of Hitbox Duration:Total Lag of Move
Extending upon this point of why sidestepping is so problematic for Marth (and by consequence, the vulnerability of waiting for sidesteps), none of his hitboxes stay out for more than about 1/3 of a second. I've already talked about nair on shield, though it is vulnerable between hits and the duration of the individual hits is still very short, but generally speaking your timing needs to be pristine. Everything lags, everything is punishable. Your "control" from neutral position is extremely fragile (and as covered, your stock is pretty fragile after that fails). It's extremely underrated to have a lasting hitbox, even if only for having a MIXUP (and in the case of the spacies' nair/dair, hit confirming into huge damage is pretty cool). Jiggs has this with nair (amazing move), Peach with dair. Sheik has her nair (amazing move). In fact, lots of bad characters even manage to have one of these. Why nothing for Marth?
Marth as a Whole
-Basically the bottom line is your advantage here is the potential to outspace other characters with range and disjointedness. Problem is that even though Marth is disjointed EVERYWHERE most good characters have one or two very good, ranged, disjointed moves that can be exploited to compete with Marth (at least force trades, which are almost universally good against Marth). He's also extremely vulnerable to projectiles disrupting his spacing.
-Grab game is great for control on non-FFers, horrifying for actually punishing or killing things. Grabs have elevated risk because you are vulnerable to the constant sidestep/wait for sidestep scenario, but imo you won't poke your way to victory against Peach and Samus so you need to get good at landing them regardless of the consequences. There is no way to be very safe here, just have to have godlike intuition.
-Big stages vs small stages: Extra mobility advantage on big stages is great, but comes at a price as getting kills will be even harder than it already is. Marth is very laggy and vulnerable to characters with better offense (fox/falco) and better stage control tools (Jiggs/Peach) on smaller stages. Kills will come easier, though. Tough call, personally I prefer big (or at least battlefield size).
-Tight battles with low/mid/high tiers. Again, all you can do is outspace. Marth's punish sucks on most of these characters and most of their punishes are fantastic on him. When sweating these out, you'll be wishing you played sheik.
-Amazing air to air game. Probably the best in the game. Exploit this advantage whenever opponents are misplaying and going in the air unsafely voluntarily. Should also exploit up throw on floaties as much as possible since you can sometimes get a **** stock off stealing a DJ and maintaining complete air control til they are launched off stage.
-Must swing sword to protect himself, but is simultaneously completely vulnerable upon whiffing. The stakes are high and the low payoff of landing a retreat fair on a floaty doesn't seem justified.
-Crappy CC game against good spacing (grab just not long/quick enough)
-Admittedly he is one of the more competent characters among the "mere mortal" tier that I would describe as anything outside of Fox/Falco/Jiggs/Peach/Sheik (Sheik is sorta stuck in the middle. Dominates the characters below but can't keep up with the characters above her imo). The problem is that there's a gap that I'm not sure can be closed between characters that play like Marth and those that play like Jiggs or Falco. Whether he is outgunned offensively or defensively he just doesn't hang and doesn't even really have solid answers imo. Then on top of that, he's still gotta worry about Sheik and Falcon who give him all the trouble he can handle, and the matchups below them are too close to just be Marth and win, you need pretty extensive experience against those characters to play them competently....much better to just be sheik and do sheik stuff and not worry about that.
As negative as this all sounds, I just want to discuss it and get people's ideas on what can be done better. I sorta only focused on negative stuff for the most part and I acknowledge that, I'm sure someone pro-Marth will fill in the positives I overlooked or understated.