With Necromancy, after activating the creature you've raised, you roll a power die. If you don't roll an enhancement, the creature dies.
With Undying, when a creature dies, you roll a power die. On a surge, it is brought back to life and restored to full wounds.
See where I'm going with this?
Does Undying work to prevent a creature restored via Necromancy from expiring? If not, why not?