[Baalz]

Taint the theme? Thematically a powerful death mage has raised many priests as undead. As I think a lich pretender is available to every nation I don't see any nation as being unthematically swayed to the dark side.

Besides, just to run the numbers, if you've got 20 indie priests thats about 2000 gold (counting the temple and upkeep). A whole lot more than that if you get a castle and more powerful priests. Now you've got to ench-7 which is probably either something you went straight for to support this strategy (big opportunity cost) or something you're not hitting terribly early in the game. Now you also need a D-4 mage and a N/W one (or something like that), and spend some gems to set this up/cast.

After all these opportunity costs you're now generating 100 longdead per turn. This is at the point in the game (after you've spent several turns actually raising the dead) where there is widespread use of SCs and battlefield wide spells.

Again, you could probably make a useful strategy with it, but I can't imagine this is balance threatening or even common.