[Cainehill]

Undead can't be affected by items giving regeneration (hydra armor, ring of regeneration) or by the normal spells for giving it (personal regeneration, regeneration, Strength of Gaiai), but if it's inherent to the undead, it'll regenerate. Similarly, Carrion Woods Pangaea can make undead regenerate (at least their own, haven't tried with a PoD or whatnot) with one of their unholy spells (Regrowth).

Regeneration doesn't do anything for fatigue, no. Different ability, given by various spells, blessings, and equipment.

So - if you can get your undead to regenerate, it's great for them. But the normal, easy, ways of getting regeneration don't work for them.