Ah well. At least we have two viable swordies.
Let's see, Meta Knight, Ike, the Pits, Toon Link, and potentially, Marth and Roy. That's 7 viable sword users with only Link, Lucina, Shulk, Robin, and Mii Swordfighter being the iffier ones.
Of those 7 considered viable even if they're barely viable, so they could hold their own, but need a secondary or something to help out, Meta Knight, Roy, and Toon Link don't play like typical sword users. Meta Knight and Roy are more like Captain Falcon, except Meta Knight is also a bit like Sheik where you need to be on point with comboing. Otherwise, these two are all or nothing fighters like Captain Falcon with the main difference being that Meta Knight's disadvantage isn't bad compared to Captain Falcon and Roy where it's abysmal. The other difference lies in their neutral and essentially, how they work like Roy's grab game isn't as good as Captain Falcon and Meta Knight who have insane dash grabs or how while Roy's neutral isn't a wacky as Captain Falcon's, having a natural disjoint helps with spacing. Oh, and these two can also speed camp you.
Toon Link's already obvious since he's more projectile reliant which does hurt him if he can't use his projectiles effectively. Kid's reliant on Bombs like Link, but lack Link's more rounded kill potential. That leaves the Pits, Ike, and Marth all of whom are more like typical sword users who space more and play less hammy unlike Roy and Meta Knight. In theory, Marth should probably be the best with his kill power and hit confirms off of jab and with customs, throw to Crescent Slash. In practice, it's probably the Pits since they're more well-rounded and deal with almost every situation well. They're the jack of all trades in the game, but surprisingly, they're not used a lot. You'd expect them and Mario to be used all over since they're that friendly and comfortable in many situations, but nope.
Link, Lucina, Shulk, Robin, and MSF suffer issues that hold them back. In Link's case, he's the heavy artillery which is great, but remember Snake? Yeah, the other heavy weapons specialist who was more explosives-orientated. Snake was heavy, he was fairly quick when attacking, and had good range for someone without a natural disjoint - let's ignore Utilt and his other disjoint shenanigans. Snake's stage control was insane since he could plant explosives, cover the air with explosives, and use regular thrown explosives like Link's Bombs. Main thing is that Snake did well at all ranges; Link does too, but there's still the issue of Link not doing well at close range against quicker attackers.
Lucina's issue is that for whatever insane logic the developers used, she is essentially a "hard mode" Marth, but they justify her lack of tippers as making her a beginner's character. Really? Then does this make Roy a more difficult character because he has to stand right next to his opponent to kill? Oh wait, almost every character can do that. The least they could have done was made Lucina a Melee clone which they didn't. She's Marth without a gimmick, but also having a gimmick. Marth's gimmick was tippers, Roy's was having strong centered hits, and Lucina? Lucina's gimmick is not having a gimmick i.e. not having Marth's tippers. That gimmick destroys any chance she has to be a well-designed character. Her attacks are only marginally stronger than Marth's untipped hits which pretty much makes her obsolete, but if you made her attacks being marginally stronger than Marth's tipped hits, then Marth is more or less obsolete. It's like if you cloned Peach and made Daisy who can't float. That's the whole point of Peach's float! Removing float would wreck Peach/Daisy since aerials are now less versatile. If Lucina was something like a "Marth who does more damage, but has no base knockback and insane knockback growth", she might actually be a character worth choosing other than "I like Lucina".
Shulk's issue is that without Monado Arts, he's Ike, but slightly slower, has more range, and sometimes, even weaker. His Dtilt for one is 3 frames slower than Ike's while being slightly stronger; 1% more damage, 25 less BKB, and 20 more KBG. The 1.0.8 patch made Shulk's dash attack pretty much Ike's pre-patch dash attack without a late hit. Ike now has a stronger, same speed, and I think longer ranged dash attack since he does an uppercut while Shulk does a swipe. Ike's dash attack also kills now. In theory, his range should be killer like Utilt? Shulk's Utilt should be the ultimate anti-air move. Main issue is that Shulk's not that noticeably strong and Monado Arts while allowing him to focus on different things aren't good when Shulk's base form isn't good. It's like if you're using a steel sword in a RPG. You can enchant it however you want, but there's a certain point where a better weapon outclasses it. Ike's good because his "base form" is good. If Shulk's base form was tweaked a little like one, a frame 3 jab for Pete's sake or more damage or knockback would help him; if Shulk's base form was good, imagine how much stronger he would be when he can buff himself with Monado Arts. Or how about this, imagine if Ike had Monado Arts. He's good, but if he could run even faster, hit even harder, do even more damage, or tank stuff even more, Ike would be a monster. That's what Shulk could be.
Robin? Mobility. Seriously, when you run slower than Jigglypuff, but you have those kinds of aerials even if they're limited, you're a good character held back by a "simple" problem. Falco, Dr. Mario, and Robin suffer the most with mobility issues. They're not Ganondorf who tank stuff, they're not Olimar or Mega Man who have really good projectiles, or even Jigglypuff who has insane air speed to counter her abysmal ground speed. Good jab, good Smashes, and usable projectiles along with good aerials are things Robin has, but are outweighed by being really, really, really slow on the ground. Last place at 1.15 for running speed, a 0.891 walking speed, and 1 air speed. Robin only has an average air speed to boast, but when that air speed is just 0.15 slower than your running speed, that's an issue. It's not like Wolf where his running speed was "bad" at 1.4, but his air speed was 1.222. Robin should at least be running at Olimar and Wolf's running speed. Seriously, Awakening did away with the weight stat and even then, WT only affected weapons you were equipped with, so even if Robin was in Shadow Dragon, those Tomes shouldn't be doing a thing if her Tome wasn't equipped. It's like saying Mewtwo's light because of psychic powers. If I had control over gravity, I would make myself heavier than Bowser and never have to deal with dying like a Jigglypuff.
MSF? You're gonna have to ask Antonykun or someone else. I know next to nothing about the Miis.