Yeah, he's not broken compared to the vast majority of other options. I have corki in my pool for adc's and he's not exceptionally hard to stop. If you dodge his Q, that's a large part of his damage missing. His R is the other skill shot to avoid, but it's not hard to move around if at far range. Other than that, his W is only used for movement (unless with package, but that's telegraphed). The only thing you can't do much about is his E, which doesn't do much damage so much as reduce defenses for his other abilities/AA's.
The reason he isn't played a lot and isn't deemed good right now is his splitting of damage between ad and ap. It makes many builds on him very expensive, pushing the purpose of him (mid game power spike) past what is viable to compete on a high level. There's just other champs that can accomplish the same things so much more efficiently.