According to the ADOM Wiki it's not clear how much, if at all, amulets of protection from undead actually protect the PC from special attacks. All that's known is it can save the PC from a banshee wail instadeath. I suggest adding some feedback message whenever the amulet takes effect, e.g. "your amulet protects you from the ghost's chilling touch", so you know it's doing something. And if it turns out to not have any benefits to begin with (other than the banshee thing), it should probably be given some.

Oh, and same goes for amulets of protection from constructs.