The problem isn't the customs themselves but rather the attitudes that act as if this is completely fine ("adaptadaptadaptadaptadapt") and people who prefer default are horrible anti-competitive people.
At what point do people just want stuff handed to them though? Ledge stalling with Villager is an objectively bad tactic. It requires consecutive (non-invincible) ledge grabs. Down smash to trade with balloons is an easy mode counter that works with almost every character. The handful who can't do that have other stuff. For that matter, there's a lot of specific things outside of this that completely shut down the tactic hard. Anyone losing to this does just need to adapt. They've been told how to beat it, it's really easy to beat, and if they just choose to lose to it, why should we ban stuff for their sake? I feel like this sort of thing comes up a lot; there has not been a single custom move that is just dominating anything. For every strong custom, we see a lot of players actually beating it but people just get hung up on the people losing to it.
Let me put the whole balance argument for customs in another way. Sandstorm just finished, customs legal. Here's our top 8:
1. Zero (default Diddy, counterpicked default Sheik once to beat Little Mac)
2. Nairo (default ZSS/default Dark Pit)
3. Mew2King (1122 default size Mii Brawler, 2312 Palutena, default Diddy)
4. SS (mostly default Ness, used some other character much earlier in the tournament but I honestly don't remember which one)
5. MVD (default Duck Hunt/default Diddy)
5. K9sbruce (default Sheik)
7. Apachi (default Little Mac, I think he was the player who used some 3111 Pit?)
7. Airvault (1123 Marth)
Without customs we can be pretty sure Airvault wouldn't have made top 8; Crescent Slash was what kept him in games and honestly is the primary reason Marth has any viability at all. Banning customs would just be removing Marth from the game and hence removing this player who was a dedicated main. Airvault's games were really entertaining so I don't see the positive to this. Mew2King got a lot of mileage out of customs with both Mii Brawler and Palutena, both effectively banned characters without customs (just transparently not viable without customs). He was not really abusive at all with either; it was just two more tournament viable characters that get to exist. I think Mew2King would have probably placed highly either way, just in a far more dull fashion probably using exclusively Diddy.
Honestly as a custom advocate it kinda frustrates me that customs aren't seeing as much play as I think they should be at big events like this (Zig-Zag Shot IMO could have helped MVD a lot). That's just a personal disappointment that they aren't making as big of an impact as I feel like they can. Either way, what problems are they creating at a large scale? ADHD got 5th at one kinda big tournament before anyone had time to lab up how to beat ledge stalling Villager (ledge stall Villager being defeated by three different characters with wildly different strategies the day it was first show, incidentally)? Kong Cyclone which seems to do worse and worse as time goes on and people learn how to beat it (though related, DK is just obviously not viable at all without customs so there's another character customs kinda add to the game)? I actually don't have a problem walking people through how to properly respond to any custom and the downsides/weaknesses to any of them, but it doesn't seem like a single one is even making itself a problem so it feels like guessing what people are upset about beyond the way early in brackets it seems pretty common for people to lose to stuff they obviously haven't seen before or haven't practiced against? Don't we want players who do not thoroughly prepare to lose? Doesn't the current evidence no suggest that customs are basically just adding characters to the game who are not doing gamebreaking things, hence clearly improving the balance (yes, I'm willing to commit to customs being more balanced than default, a lot more balanced in my opinion)?
This isn't an anomaly; customs have been legal at a lot of substantial events. How many events has custom Villager won as in taken first place? I have never seen custom Villager win a tournament; maybe he has won some smaller, less publicized ones (I'd love to see the results/videos from such events, if anyone can actually point to one). No amount of determined Kong Cyclone use seems to be propelling DK into tournament winning either; he's made quite a few top 8s but never actually won with no consistency at all as per which characters ultimately take him out (suggesting to me that every character can beat him). I don't think the pro-custom side is being unreasonable to say, that with customs having been legal at so many events, that we want to see actual results out of allegedly broken things.
I don't think the anti-custom people are awful people, but I do think their cause is misguided. Banning customs is effectively banning a lot of characters who are only viable because of customs and is greatly reducing the gameplay variety of the game. Players who have put in the time to master customs and master gameplay elements only present with customs legal are basically having "their part" of the game banned. For however annoying anti-custom people find playing against customs while still being allowed to use everything they've practiced, it's at least twice as soul-crushing to pro-custom people to have to play in events with their mains banned and even those of us who main characters who are merely gimped by non-custom instead of completely ruined it's just dissatisfying since every time I lose in customs off I wonder if I'm really the worse player or if I'm just being held back by not having my real character. I feel like just how awful that is for custom players far outweighs any annoyance the anti-custom crowd has to suffer for playing against something they don't like. I feel doubly strong about this given the current state of affairs that has clearly continued post-patch of default characters winning custom tournaments.
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I'm sure many are still not persuaded. Let me double down. I'm 100% convinced customs are not a balance problem and that no specific custom warrants a ban. I want to hear the other side get specific here. Name specific customs that are broken. Name specific match-ups you believe those customs render unwinnable that are otherwise winnable with customs banned. If you think a specific custom is merely a matter of gameplay degeneracy as opposed to match-up breaking, explain in fine detail how that's the case and specifically why no one can do anything about it. I don't mean to just make a burden for "your side"; I just want a series of falsifiable claims that hinge on objective game mechanics since claims like "some customs cause problems" are just impossible to argue against by their very nature. I can promise my efforts and I'm sure many other pro-custom players will also lend their help in showing specifically how any specific case isn't a problem. If we can answer every specific case, there should be no problem. So, let's have at it?