It's a case of arsenal vs. experience, and I think that unless the Nerevarine had all the powerful artifacts he collected on his way, his experience wouldn't be enough. Sure, he defeated a demigod, but the Dovahkiin defeated an actual god and dozens of dragons, AND has access to their power!
That said, the Nerevarine could have easily ended the Oblivion Crysis, the Champion of Cyrodiil was a mere mortal with skill - that's it.
Ending the dragon crysis is a whole different ball game, but I think he could find a way.

Nerevarine was just about to land the killing blow but the screen froze! :brokencomputer: