He's got two specials that are really similar. Even the start up is hard to tell at a glance and need different reactions
His normals are a mix of "I do an awkward spin" and "I do awkward punches and kicks."
He's always been a lame character to fight, but especially so in high-latency Ultimate.
He was a last minute addition to Brawl and had a rushed moveset because of it, but they never went back and updated it.
He has never really "felt" like his source material. Sonic actually takes a fair amount of momentum to get going, but then is hard to stop once he's moving. In Smash he just ZOOMS.
Speaking of which, he barely takes enough of his source material. He's very loosely inspired by 2D Sonic games, plus the homing attack. Where's the bounce or light dash? They could probably give him some newer moves, too, like a sword or werehog, but that wouldn't be received so well.
There's some other characters I think have a consistently poor implementation (Kirby, Little Mac, Ganondorf, Zelda), or a weird one that ends up working (Link, DK, Luigi), but Sonic is easily the worst IMO.
