Well, some guys said that they didn't like the crosshair. Others didn't like the fact that M4A1 has no silencer. Others said that aiming with sniper rifles is hard and others didn't like the spray with guns like P90.
I have played the game and for me, the only cons of it would be the M4A1 which has no silencer. Only that. The game is good. It's all about time and exercise.