I don’t mind if your accuracy goes down when you go full auto, or your aim point drifts in some random direction, but having to scroll down as you shot just sucked.
It's not even that realistic. In real life, you need to apply constant pressure downward to keep a machine gun firing level, but not constant movement.
IMO, a realistic way to do it would be that when you fire the game machine gun in full auto, your point of aim gets shifted upward. To return it to where you attempted to aim, you'd then have to move the mouse downward slightly and then hold it there for the duration of the burst. (And the viewpoint/point of aim should suddenly dip back downward when you stop firing.)