The main thing with the Sheaim is that you can't build any standard units beyond Pyre Zombies, but get them through Planar Gates instead. Calabim, once you have several large cities, some level 6 units (or level 4 Moroi), and a single Vampire unit (Champion UU), basically become a juggernaut. Not much that you can do to stop an army of high level regenerating, hasted, death II casters.