clears throat
http://ci.memecdn.com/10158912.jpg
Anyway, Corki suffers from pretty high mana expenditures, forcing him to invest in mana early on, or have to play more passively in lane. As for other drawbacks, he's one of the few ADC's who lacks CC, and his only escape is on a 20s CD, at rank one.
For the reason why you can knock him back during his package dash, I don't know, I don't work for Riot. If I had to hazard a guess, its to ensure that the effects go off correctly, since it would be strange if you knocked Corki back, only to have him knock you back, followed by a phantom trail of slow/damage, where Corki used to be. However, that's just a guess.