With Bannor you want Cottages everywhere, so Hell Terrain makes little difference to that, unless you're on Flood Plains. Still since the Bannor have been fighting hell since their creation, it makes sense thematically that HT wouldn't slow them down very much. Still, its not like its a bonus to them or anything.
Well, getting the health hit from StW does slow down Bannor quite a bit. They do not get magickaly appearing units through gates like the Sheaim. So, they have to rely on a healthy city size/production combination to get the more useful troops, for producing enough demagogs, and for having enough people to work on those villages so they become towns again to produce more free demagogs.
Demons are the ones that get a bonus on Hell terrain, and there are a few AV units that are demons. You'll want to bring someone who can cast Sanctify with any stacks anyway.
A list of AV units may be useful here. What I can remember right now is:
1)Diseased corpses(Undead)
2)Eidolon
3)Beast of Agares
4)Ritualist
5)Savant
6)Their High Priest, can't recall the name
So, unless the Beast of Agares is a demon, I do not see any other units in AV being demons... And, besides that, Demon Slaying gives Order units +40% against demons, while they just get 10%(IIRC) on HT.
The sanctify may be necessary if you run Order, but not with AV. Besides, HT, slowly reverts back to normal when it gets to Order lands.
EDIT: This can only happen if you conquer an area already having HT, of course, since HT cannot naturaly spread in Order lands