Why does Desert Tombs suck?

Try playing a Blessing Effect with your Tomb Wyrms and using your Death Gem income only for Tomb Kings. If you can make your Pretender a SC and speedresearch to Enchantment 3 so you can take indy provinces with 2 or 3 Sauromancers each. Pick a low price Castle so you can build them everywhere to make Shamans and Sauromancers, forget about a normal army.

That's how I enjoy playing DT. They do very well later game if you build yourself up a decent Tomb King population for reanimation. But they are weaker than the base C'tis and Miasma. If the cost was lowered by half, you'd have much more magic you could put into a pretender in order to counter some of the weaknesses of DT.