It doesn't give you their abilities, you only appear to be that race.

Fane designed the mask to blend in, which is why I am guessing it is only available to Fane/undead. You are only wearing their face, you do not become a different race. Now it does beg the question, how does an undead dwarf get away with wearing the face of a lizard or an elf...? If I am not mistake you need to get 4 faces of each particular race and then you can make a face (16 face rips). Or maybe you just need 4 faces of a particular race then you can wear that racial face.