Only real problem is the gunner getting no cover what so ever. My experience with using the turret in a LAV seems to be that you take more dmg from getting hit than normal, like every thing counts as a head shot. That said a shield based heavy would probably make a good turret user. They could survive glancing blows easier and recover on their own. If the fit would allow it I was curious if a missile turret wouldn't be a better choice. Less accurate but it has a blast radius and tends to one shot most suits if it gets a direct hit while the rail gun can take 2-3 shots to kill infantry.

If your just firing the turret as the LAV zooms by I would of thought the rail turret be really hard to hit with.