[Imp]

Limitations of the game system, crewed weapons are vulnerable as considered grouped round the weapon rather than dispersed as a squad could be.
Units can only have one state so as most of their time is spent stationary manning the weapon thats whats modeled.
They are very vulnerable therefore when moving or transported.
Not unfeasable as again restricted only that crew can man that gun.
If fired on by a sniper its fair to assume the crew would disperse to save themselves at least till the threat is neutralised, in game terms its out of action thats a kill.