I would think that Ragnos' power was enough to terrify the duo, and Nadd was much weaker than either of them. He would be like adding a youngling to Obi and Anakin versus Dooku on crack. Either way, he held sway for a long time for some reason. For all we know, he could instakill Force users like some people blink. To die on your deathbed unmolested means you are damn hard.
And I noticed I omitted Tulak Hord, who is perhaps a better lightsaber duellist than either Nadd or Exar Kun... but again, he's hard to argue for since all he has is a reputation.