I've fought against custom Sonic a lot. Sonic remains a top tier character and both of his side customs are pretty awesome, but they really aren't unfair in any way. Hammer Spin Dash still loses to the "hit Sonic out of it" strategy, and of course, it can only ground you if he hits you on his way down if you're grounded. Burning Spin Dash is a lot less flexible of a movement/pressure option in exchange for a lot of damage and the ability to plow through some projectiles. I feel like a lot of the Sonic hate is misplaced; you have to play a different game when Sonic is involved, but that different game is really fun and really interesting. Customs only enhance this in my opinion.
I'm sorry Shaya, but I just don't agree with your assessment of customs promoting killing at 60%. You pretty much only die at 60% if your opponent makes a really huge read (like Shulk in Shield or Smash using Power Vision against one of your smashes) or if you make bad decisions like taking Mii Brawler to Smashville and then going out on that platform so close to the blast zone. This sort of bad decision making also happens without customs; people do foolish things like chase Sonic to near a walk-off and get back thrown and die at any percent or people let Diddy or Rosalina grab them on high platforms which can only happen if you don't consider it an urgent problem to avoid those situations. Certainly customs add a variety of excellent kill moves, but kill at 60% is hyperbole if all of the players are passably well informed and make decent decisions. I challenge anyone to find a stream archive of this game, customs on or off, in which between losers/winners/grand finals half of the stocks or more are taken at under 70%. That's just not how things go, and I do not believe they are more likely to happen that way in the future either.
I also think that the way that the game is way more balanced with customs on is about more than Diddy. Like Diddy is a really relevant factor, but even if Diddy didn't exist, the customs on game would still be a lot more balanced than the customs off game both because the inverse relation between general character quality and power of customs is pretty strong (characters like Wii Fit Trainer get to be viable; that's a lot of help) and more importantly because everyone having better options just makes match-ups tend toward being more even. When everyone has options, the game is more dynamic and you can respond to more situations. When options are more limited, that's when you get the situations where someone just gets locked out and you see your awful match-ups.
For the most extreme case, consider what @
Ffamran
said in jest except take it seriously. Let's say Falco were compelled to run 3312 in tournament but everyone else were compelled to run their single worst custom load-out as well. As Rosalina, I think my worst would be 3132. Imagine if we had to fight each other, super bad Falco vs super bad Rosalina. Sure my ability to deploy Luma and control remote space goes to about zero with those awful Rosalina customs, but with those awful Falco customs, his ability to get past a basic "zone with normals" game from Rosa is going to go way down (he loses everything he had on the B button that might have helped!) and that's going to make that match-up awful for Falco in a far more painful way than any benefit Falco might get from the significant nerfs Rosalina faces from those awful customs. In this silly hypothetical metagame, we might expect match-ups like this to happen a lot, things getting lopsided just because one side's option pool is just too dry.
I can't deny that preference is preference, but at that point, you're moving into the subjective. No doubt some people (by most evidence about 20% of players) just find customs off more fun. That's fine; I spent years finding Brawl a lot more fun and generally "better" than Melee so I'm not really in a position to tell people that not believing what the majority believes makes them wrong. I think the important thing is that we remain respectful of objective truth (claims like "customs are broken" are, with our current evidence, objectively wrong), remain understanding of subjective preference, and we all do the best we can do to promote whichever very specific game it is that we love. Smash 4 with customs in the game I play and the game I love. I'm here to help that game and to promote it however I can. Definitely I would prefer to avoid bad arguments and to focus on building the meta; a good starting point might be, to directly address what Shaya is saying, figuring out what average kill percents actually are in customs on and customs off and seeing how it actually varies character to character. That would be really interesting actually, and having a discussion with numbers might make it a lot more enjoyable for everyone.