Both. Some higher level and named mobs are immune to various types of damage. The smart 'Zerker will carry several types of weapons and be prepared for any fight.
So far I have only found 2 mobs that resist a certain type of weapon and it was pierceing on both of them. One of them was the named crab by the beach in antonica (the beach closest to the claymore). The other was a lizard in the well inside stormhold.
Jah, its mostly the upper level raid mobs that are immune to certain types, some of the higher ones (theres one in feerott I think) that are immune to 2 types. (That one is slashing and peircing if I remember right.)