NC Soft seems to make a it so that anything undead can't be killed by those with undead casts. And if it can be killed, it's changed to plant status, as in C3 with EG trees going from undead to plants, in fact all undead trees went plant. Paladins are obviously excluded.
"Sacred" attacks still affect only Undead mobs... not "Dark" types in general, to include Holy weapon.
I always leave HW on as it doesnt significantly affect my MP regen and helps against the Sacred-weak Cata/Necro Mobs... but it's generally pretty useless.