What I see is the increase in the amount of damage it can take. One well trained techie gunner can't kill it in one mag if its moving. Also, the damage RPG and SPG shots cause on the BH is infirm. All times I shot an RPG at the BH, hit it then it ran away. I don't think its a bug, but rather the increase of damage it can take. I think IRL if a HEAT RPG hits the BH, it would go down, unless it hit some unimportant area. Same goes for SPG. I managed to get one HEAT SPG hit on the BH and it kept flying without a problem. I also tried a Frag, with no difference.
I have some screens for the RPGs on BH, but for the SPG, the BH was close and too fast, and I did not get a screen shot in time.:


(notice the smoke cloud generated by my RPG when it hit the BH)
Also, a tandem shot seems to immediately cripple the BH(it catches fire), but I don't think I have a good screenshot of that.





