The ATank ones, depends on the map. Some are wide open and you will get sniped in seconds. HOWEVER. Nobody jumps on one to take out a tank and then jump right off. It's easy kills and helps your team.
The machine guns with the gun shield are pretty effective against snipers, UNLESS they are to your side. You have to know the situation first. Also a UAV is EASILY taken out with stationary MG. You should be able to down that think in 5 seconds. Hell any LMG will do the same.