Both factions are one-way trips to Ally. There's no mob I know of that lowers either faction for killing it, or tasks/missions/quests/etc. Further, the only mobs to kill that raise these factions are the instanced targets that require some raid that is already 85% Ally. The factions are entirely task-driven.
There are no negative effects for either faction going up.
The other factions that matter to 75+ (especially "+", more accurately "++") raiding are Crusade of Scale, Wraithguard, and Wraithguard Leadership.