Is Quinn balanced?
Quinn is a ranged ADC who has the damage of an ADC.
However, unlike other ADCs like Jinx and Ashe, Quinn is also more tanky and has the means to peel for herself with blinds and knockbacks. Arguably she also deals faster damage than other adcs, which gives her assassin level burst.
Personally, I've never seen a Quinn go negative KDA in any of the games I've played in before, and I think a huge part of that is due to her self peel abilities. I think that this minimizes her risk factor while keeping her reward factor high.
If you're an assassin, let's say a Katarina, against an Ashe solo farming in bot lane, you can take advantage of the fact that she's out of position and go in to assassinate her, which is what an assassin should do. However, if it was a Quinn, you could Q, then shunpo in, and Quinn would blind you, knock herself back and then crit you 3 times in a row before you could do anything else. I'm having trouble seeing what Quinn's weakness is.
Graves and Draven have this problem too.