i see them perfect in an undead army, be it sandy or swampy.
Even vampire spawn in such large units seem ridiculous. Maybe if they had skirmish it would make more sense, give it a more bestial feel.
Only vampire knights should be a rankable vampiric unit imo.
For me it should be more about vampires enthralling alive or dead prey, not an assortment of magical beasts that have no other history in the world.
The shrieking horror, ghouls, ghasts, winged reapers and vampire spawn all seem to break this logic.
I do however recognise that my opinions are uniquely my own. And as acs I must represent the whole community. So don't expect some mass exodus of models from the book.