I would agree that Sonic could be some wacko character stan that makes it harder on themselves by playing characters that suck, but the proof is in the pudding. No one thought Rain was good until Sonic won with him. No one thought Sindel was good, and now here we are. Thought Baraka and Cyrax were busted before anyone knew that. Called out Smoke as mid tier while everyone thought he was broken.
I'm not saying it's impossible that Sonic's take on a character is wrong at some given time, but you're talking about the best to ever play the game, and someone who literally dedicates their life to labbing and competing. Your first reaction when someone like that says a character is strong should be to assume they're right and try to get to the bottom of what you've missed before saying "no way, the GOAT is wrong".
And if the argument is that a character is only strong in Sonic's hands, well, that's not really the character's fault, nor Sonic's. A "tier list" to me means the upper limit for a character compared to other characters in a competitive setting, not an indication of how easy it is for the community to figure out and use broken stuff.