I don't play Underworlds as I don't like the idea of deck building, as it is very unbalanced. It just doesn't seem like a fun experience. If I want to play a board game of AoS I'll play Wsrhammer Quest.
But, I have a bunch of underworlds models in the armies my husband and I collect. But, we generally don't use them with their underworld's scrolls and instead use them as counts as.
Crimson Court: Use them as Vampire Lords. I'm considering using Duvalle's scroll anyway even without the add ons and lowering the points a bit, only in friendly games of course.
Hrothgorn's Trappers: Use as a hunter and a Sabre tusk
Morgweath's Coven: Use as a Hag Queen, Witch elf champion and Sisters of slaughter champion.
Briar Queen: Actually use the Briar Queen and Thorns AoS warscroll for this one. Nighthaunt is so low on Wizards I think it's worth it.