That's a good point about characters that are more able to avoid gimping. Which characters are those? Pit's got ungodly recovery, but he goes into freefall if you hit him during his up-b. Metaknight's got literally unlimited recovery, but he's so light that it seems like it would hardly take more than one well-placed homing attack to knock him out. Snake I can see being a big issue, along with Luigi. Maybe DeDeDe as well because of his heavy weight, multi-jumps, and really, really nice up-b.
All in all, I think that Sonic's reliance on gimping makes some characters really good counters for him, but I don't think that having a few good counter characters warrants declaring a character sub-par. Hell, I'm a Luigi fan through and through; the fact that the Spacies eat him for dinner doesn't make me see him as a worse character in my eyes, it just makes me not use Luigi against the Spacies.
I do see where you're coming from though. Are there any other nigh-unto ungimpable characters?
snake is probably his biggest promblem... look at this.
-snake is pretty much unnable to be killed by sonic through standard means until about 200 damage
-snake is very difficult, if not impossible to gimp.
-snake can kill sonic at sub 100% damage.
-snake has higher priority, longer range and stronger moves.
He doesn't just counter sonic, he shuts him down thus far.
as for luigi, it's sometimes possible to gimp him by walling his FB... but one missfire has the ability to kill sonic or at least get him away... and luigi should be countering well enough on the stage that sonic doesn't win this matchup.
kirby is ungimpable, and just about as good as luigi on the ground.
metaknight is almost impossible for sonic to get inside with his crap priority and metaknight's disjointed hitboxes and small size. add the fact that he's hard to gimp and sonic ain't got much he can do. I mean, it doesn't take a lot to kill him, but the problem is that he's REALLY good at stopping sonic from even doing that little bit.
Dedede is actually not a bad matchup for sonic... I mean, it's not great, but sonic has a shot against him for being so big and slow.
sonic's homing attack BTW, is easy for most humans to get around. and it doesn't pack the punch you want most of the time. and the spring drop (I refuse to call it a spring spike becasue it doesn't spike at all.) has really bad knockback... so in many situations it won't do anything to stop a character from coming back... the exceptions are hitting early against the character's up+B or hitting when an enemy is right under a stage lip and so bounces off for a spike effect.... which can also be acheived occasionally with a homing attack... because of this, I'm thinking that stages with lips might be good stages for sonic to choose.
but, looking at the tentative teirt lists, it looks like most people at the top are saying that sonic is in the lowest teir possible... I'm hopeful we can dig him out, but I mean pros are seeing NOTHING from him. I'm kinda curious as to why this is because, I mean, they are the cream of the crop, you'd think that they'd find some of the things we did and wouldn't put him tied for dead last.
edit: I'm not sure how much I need to be telling you this, but if you've played brawl, you've probably noticed that gimping in general is MUCH more difficult than it was in melee. it's not near as much a part of the gameplay.
