> Implying 64 balanced
That's besides the point I suppose.
One of the problems I take with boost-dancing is that it's dash-dancing if dash-dancing were made as stupid as possible, and dash-dancing is already really really stupid. While smash isn't a traditional fighting game by any stretch of the word, it can destroy a lot of low-tiers on its own (as well as numerous aspects of an MU being reduced to "dash-dance harder") and make even high tiers seem unviable... to see what I am talking about, go watch Armada vs PPMD winner's semis at APEX 2015. PPMD literally only loses game 3 with Marth because he made a mistake and Armada executed a zero-to-death, but PPMD makes Armada's Peach look like a n00b-level Peach who thinks dash attack is the best move in the game and dsmash is a miracle from heaven. Boost-dancing takes that up to 11 in such a manner that the only way to reliably hit Sonic out of a boost dance is to swing for the fences... and he can do this at any point on any stage by just going up vertically and then boosting until he gets center stage to work his magic. Worse, it completely takes it covering landings out of the equation in a smash game where that's rather important for some characters [even Pikachu's landing can be covered, but boost is impossible to cover Sonic touching down].
You've thus got an unhittable, unjugglable foe in the right hands, which is, as Mawtooad said, fundamentally broken game design.
Now, it's not EASY to do this, don't get me wrong, but neither is Palutena's infinite lightweight glitch [or rather, actually controlling her during lightweight], yet I met a guy who was nearly perfect at it. My best bet in combating this was one of two options - pick Sheik and spend the entire match fair walling, then look for a bouncing fish or one grab to try to uair her [only workable because of how absurdly safe Smash 4 Sheik fair is], or pick Pikachu and spam back air because Palutena can't grab Pikachu's landing animation on bair, and her dash attack goes over it as well, leaving only super speed [inconsistent] or jab as punish options (After enough bairs, fish for usmash, fsmash, or heavy skull bash KO). The former is tedious and annoying [one mistake can spell death], while the latter is interactive in only the dumbest sense of the word. We played for several hours, and combating the man who could zero-death you off an error was frustrating difficult.
So what does Sonic have? A get out of non-perfect combos free card [boost], a land for free card [boost], and a I'm-even-harder-to-hit-than-Melee-Marth-and-Lightweight-Palutena-combined card [boost], all rolled into one move. It's poison to game design to make a character with a move that means they should fundamentally never get hit, as the rest of the cast is just too slow to keep up with them [if you can't shield out of boost, this alleviates things VERY slightly since projectiles can do a little to mitigate, but they have startup and Sonic's boost can punish anything crosstage in less than a second, so that's of dubious value... and I think he can shield out of boost, so just press R then spring out of shield and go back to boost dancing].
And quite frankly, the problem was NEVER losing to a Sonic - I played Kien and almost won just spamming Vanish [back when Vanish was STUPID broken], and have taken games (and lost them) with Falco, Sheik, and more. The problem is that this was occurring in a super laggy Wi-Fi room with a rusty Kien - if you make this game competitive and give someone who really likes Sonic a year to really nail down the intricasies of boost, they just shouldn't get hit, because they never have to commit, ever. And that's just bad game design.
If Sonic is getting walled by Marth or armored through by Bowser, he's doing it oh so very wrong. He can just boost dance around Marth's short-duration hitboxes and wait for a pattern, then strike [or hit him once and run away for 8 minutes], and he should be grabbing Bowser nearly every time. Falcon shouldn't be outmaneuvering Sonic ever.
And if people are gonna say "Well, why don't YOU break the character?" Frankly, it's not fun. I've never enjoyed playing Sonic in any game. If people really were convinced Sonic is bad [I think he was the theoretically best character in every patch with unlimited boost, although he had an extremely high skill level required to make that happen] and I had my setup on me, I'd be willing to try to break Sonic's boost enough to get him nerfed if that's what it took, but the idea is thoroughly unappealing, because attempting to show why a toxic character is toxic is not something that's fun to me.