The undead are no longer particularly effective against the Ashan factions. They are dead and long cursed. I don't remember a time when the undead were considered holy.
We could argue for a long time about how vulnerability to "words" is distributed in Ashan. But we didn't gather for that.
Light magic comes from Elrath and is closely related to the ideals of truth, justice, and perfection. Light shines on the virtuous, but blinds and burns the unrighteous.
dark elves should be vulnerable to the word due to their connection with darkness - it's like water and fire, they destroy each other and cannot exist together.