The Ulti isn't the reason she's not a super strong AD carry. And it can't be interrupted with just an autoattack.
The reason she's not as strong as other AD carries is she's got Vayne's rather ho-hum range, without the damage you get from other champions with that range, like Graves and Vayne, and she has no escapes other than an AOE slow and her passive.
What she needs is a stronger steroid.
I mean, Vayne has her silver bolts, which do percentage of maximum health as true damage every 3 attacks + a flat amount. That is preposterously strong. And her ult and Q are both steroids, and her Q + E is a very strong escape, or Kill Securing form of CC.
MF's impure shots do a small amount of magic damage every shot, and she's got an AOE slow. Ho-hum.