There is nothing wrong with Graves as an ADC.
His problem comes with his ranges. See, has auto range is one of the shorter ones (more in line with mages than other marksman)
His abilities are -
Q: The three shots.
W: The the smoke launcher
E: The Dash
R: The massive aoe line.
So, with that in mind, pros and cons.
Cons:
Short Range, only one 'real' damage ability, that needs you to be close to the target.
A smoke launcher, that to be honest, doesn't really do anything, except prep for ganks or escapes....and vigilant pings for the sight blocked enemy thwart this.
The Dash is NICE - but he needs it to engage, due to his range....so he generally doesn't have an escape.
Pros:
MASSIVE damage with that Q, if you can get the damage items.
The smoke launcher does get people to do stupid things in team fights, and that's pretty funny.
Technically, he is a mobile champion.
His ult, with just a few items, can auto-win team-fights.
So overall, Graves is strong. But Champions like Lucian, Tristanta, Kog Maw, and Cait (most of the current meta) roll him with their range/better mobility/both. Seeing as these are the current meta (practically) it is fairly risky to play him. Worse, the Carry's he does best against dont see much play (some people have dependencies with me over if Lucian is a counter, but in general, Lucian seems to win that lane)