They don't have much more HP than baal or ubers
I mean I never had a problem with blood boil even though its not that suited for bosses, because they also come with 14+ corpses to engorged, but that's warlock OP
killing mercs is indeed a real issue for chars like javazon/light sorc reliant on it, otherwise heralds can often just be big damage sponges that you can trap behind some geometry and rail down. A fire sorc spamming meteor+fireball from across a gap doesn't have much problem dealing 2million+ damage.
Yeah nova seems to be a tricky one due to the range needed I’ve used the geometry and doors before but man still hella annoying with how soft they are