I'm genuinely curious lol
Disjoints? I know a lot of Sonics think he loses to Cloud so I guess swordies can give him problems
Trouble taking the stock
Non-existent landing options
Lackluster neutral (no options to pressure shield with, no good autocancels and all his specials are negatively disjointed, so you can always hit him before he hits you. "His so-called neutral" is basically whiff-punishing. He has to outplay in your game if he wants to push to advantage)
Gets walled out pretty hard (which is why Marth, Ike and TL are hard MUs even though they are winning or 50:50)
If camped, he can't really do much against. This is because of lack of a real approach. Everything has either too much startup, not safe on shield or gets beaten out by other hitboxes)
Underrated (in terms of how bad it is) disadvantage state (Bad air speed, no landing options and no real combo breaker makes it that Sonic is meant to be punished hard for messing up a SpinDash play. He just gets too much respect in disadvantage thanks to everyone believeing that Spring is this godlike combo breaker when it actually puts him in a worse position.)