Yeah, the one problem with the template system is that the demilich template doesn't requrire the experience or gold costs required by the class, so there is really nothing stopping any evil spellcaster from taking it and then wreaking havoc.
That said, "overpowered," as I have pointed out in other threads, is largely a question of which module one is playing. I would never be a demilich in a module the character could then just slash through with no challenge, but in a module with brutal scaling, like Endless Nights 4, I have played a demilich without feeling that choice made the module too easy. The mobs get huge in the latter portion of the module, and even the best wail of the banshee won't kill everyone. Strong enough opponents, such as giants, can do some damage against the template demilich, as can opponents armed with weapons that do divine, magical, or positive energy damage. Also, the fact that the PRC hasn't quite worked out a way for demiliches to heal themselves with harm, it is hard for a demilich to heal damage in the way normal characters do. Healing kits still work, though they shouldn't under PNP, but they don't heal as much damage as a regular character could heal with a heal potion or spell. As a result of this problem, the demilich isn't as overpowered as it might at first appear. Hypothetically, someone with immunity to death magic and a really good weapon could bring a demilich down. Five such opponents would definitely prevail.
