My issue with allowing custom Mii Fighters revolves around the fact that Mii Brawler's custom Up Bs are really kinda absurd. I'm open to allowing custom Miis if Mii Brawler custom Up Bs are banned, if not I'd rather see all default (which is simpler anyways).
Sure, so long as we ban Zero Suit Samus while we're at it. Her helicopter kick can kill about as fast as Brawler's, and she has ways to kill sooner than he does.
No, I don't seriously expect us to ban ZSS. I'm just pointing out that there are characters capable of doing Brawler's little early kills trick even better than Brawler.
Beyond this, I am going to present an argument in favor of free design for Miis in tournaments.
Simply put, limiting the Miis to 1111 makes no sense because there is no indication those are the true 'default moves.' Bear in mind the Mii's entire purpose is 'create your own character.' When you try 1111 loadouts on the Miis, you quickly see that 1111 is not even a coherent movelist on some of them.
Brawler suddenly can't kill well and has no viable recovery. All he gets out of it is an unimpressive projectile that does not compliment how he should be fighting to begin with. Brawler goes from being a threat, to having to fish for random f-smash kills because everything else he does is not much of a threat since he has no real mix-up without threatening air kill combos off a grab.
Swordfighter likewise loses a lot of recovery, as Stone Scabbard is very vulnerable and Aerial Assault can simply be 'body blocked' much like Ike's side-B in Brawl could be. Not quite as effectively as in Brawl Ike's case, but it is do-able. Likewise he finds himself limited to a close range projectile and close range counter, gutting the versatility that is supposed to be the point of the character. In combination, these 'default moves' present a game plan that is hard for even the Swordfighter player to discern; it's not clear how he should safely recover, it's not clear how he fights at long range.
Gunner is the only one that makes any sense with 1111. There at least I can see a coherent game plan; Charge Shot to kill (in theory. In reality it hits far too weakly to do this), Flame Pillar to control space and disrupt approaches, Reflect for obvious reasons, and Up-B 1 isn't a good recovery but it is at least okay.
Still, in the case of 2 of the 3 Miis you look at 1111 and ask "Okay, what does this character want to do?" I have trouble believing they were balanced around the concept of 1111 in their design and testing phases of development. Brawler becomes just short of worthless, and Swordfighter gains massive flaws. Gunner's gameplan is at least coherent, but I would not call it good.
I strongly believe Smash 4 is better off with the Miis being free to customize in terms of height/weight and movelist. Not equipment, just those variables. I urge tournament organizers to allow more freedom in Mii design, and online ladders to do the same. '1111' simply hurts these three characters far too much, and there is no clear indication they were designed around 1111 since they ignore the Custom Moves Off rule unlike the entire rest of the cast (including Palutena).