There are issues when one uses enablePersonTurret script command to disable a FFV position.
For example, on Offroad car (e.g. I_G_Offroad_01_F), if you enter "Passenger Right Seat 1" or "Passenger Left Seat1" (turretID 2 and 3 respectively). If you disable person turret, and then turn out, the Character points forward (check external camera), but the internal camera is locked to pointing backwards.
It seems the animation executed during the turnout move does not apply to the camera.
Also, the position of the animation may completely change - possibly due to proxy animations not applying. For example on Mora APC (I_APC_tracked_03_cannon_F), the commander sits a lot lower if you disable FFV - only the head shows, the arm glitches through the vehicle. It may not be the worst here, but it can be depending on offsets of proxies between turned-in and turned-out state.
In a custom vehicle of mine, the crew ends up sitting 2m above from the position it would have if FFV was enabled.