they are very nice
you should try blood Lach
there you get vampire counts they are immortal too but even a bit more useful than the demiliches . and they can summon 1 vampire / turn which is also immortal .
a main advantage of the demiliches is ihmo that they are death 4 so they can cast drain life without items/empowerment.
the vampire counts have death 3 blood 3 instead so they can't cast drain life without items but because they summon vampires you can mass quite big numbers of them and they are flying !