Weight. Guns and bullets are heavy - really heavy, especially if you want to penetrate even light body armor.

I was going to say recoil, but ironically I think that's the easiest thing to control for with software. It's predictable, and modern drones are highly maneuverable; it wouldn't be hard to have the drone software account for recoil when firing bursts. It'd be interesting to see just how accurate you could get shot placement.