Genji slaughters most heroes as he's border line OP but McCree is one of the few that might actually stand a chance if he gets the stun off. Something thats not that hard to do. He also wins over Genji at all ranges where his stun does not reach as his shots are hitscan so you should never miss a shot where Genjis projectiles are slow as heck, fortunately.
Then again, McCree can barely move so all a Genji has to do is walk away.