I was testing the underbarrel grenade launcher and c4 to take out sundys. Shouldn't 2 grenade shots at the rear and 2 c4 take a sundy out? Was this changed???
Nope. It doesn't. Why? Because some might consider having the ability to instagib sundies by yourself while flying to be overpowered.
In one of the recent updates they reduced the amount of damage sundy's take from C4 a bit, probably to stop this exact thing. They increased the sunderer's red zone though so that two C4 still puts it in the critical zone where it starts to burn down on its own if not repaired. I think you can still instagib with 2 c4 and an AV turret round, but if you're going to be engy three tank mines does the job easier anyway.
Does the armor on a sundy effect damage from UBGL? Sundys are strongest in the back right? Where are sundy soft spots?
No soft spots. The sundy takes the same dmg from any angle it is hit from. The only way to make it so there is less dmg to the rear of the vehicle is if you have blockade armor equipped, then they sundy takes %5 less dmg from the rear than the rest of the bus.