Exar Kun, I feel if he was a better tactician like Thrawn, Ackbar, Tarkin, or Revan (I mention him reluctantly to you) he could have conquered the entire republic and wiped out the jedi order considering power wise he was so ahead of his time. I think its safe to say( regardless of if its official canon or not) he is in the Top5 most powerful Sith Lords of all time (for me around 3-4).
I think Exar Kuns mistake was that he didn't expect Ulic to turn back to abandon the dark side. His mistake was similar to Tarkin's mistake in the end of Episode IV. He was not as good a tactician as the three first and he wasn't as good as Revan in planning long-time strategies, but he could still have conquered the galaxy, I think.
Wiping out the Jedi is impossible, though. Just as wiping out the Sith. They always return somehow...
During this time, Exar Kun unleashed his Terentateks on Jedi occupied worlds of Tython, Kashyyyk, and Tatooine. These beasts fed on the blood of Force users and proved an incredibly dangerous foe to the Jedi Order.
Hmm, Tython. Wasn't it lost at that time?