Are some heroes unrezzable

[unholyskipper]

with the undead rezzing everyone cant a fallen hero be returned to life? It might not fit lorewise but according to lore undead do rez regular soldiers why not bring back major heroes. I know this goes against progression and all but im wondering are there certian reasons different wow chars cant be rezzed.

by hero i mean somebody like cairne or uther ... or others who died

[Overlord DM]

I would like if they could manage to bringe Grom Hellscream back, but rezzing is kind of dishonorable :/
Seeing as the generic dead human makes a great rotten meatshield or canonfodder, a honorable hero such as said Cairne or Uther would just feel wrong.