SunRazer
Back From The Dead
After being told about Nihilus' devastation of Katarr, the Exile can choose from a variety of options, such as:
"I don't understand how he hasn't conquered the galaxy with that kind of power."
"How can anyone have that kind of power?"
"His power is difficult to believe."
"How is he able to perform such slaughter?"
"That kind of power is impossible."
Given that the Exile has already been in Revan's presence before and the game suggests that the Exile knew of MW Revan's powers in person, it's obvious that this means that she thinks Nihilus >> Revan, since she doesn't even know how it's possible for Nihilus to be so powerful.
And when you fight Nihilus:
{Gameplay Programmer: Nihilus turns, raises his hand, and the entire party is raised into the air, similar to Force Crush. He speaks, his voice roaring, and the player should realize that Nihilus is FAR more powerful than they had believed.-- Knights of the Old Republic II: The Sith Lords
Just as icing on the cake, the Jedi Masters obviously knew of Revan as a Jedi in person, and they fought him as Darth Revan (even capturing him), yet they say this of Nihilus:
At the last Jedi conclave on the Miraluka world of Katarr, the entire planet was wiped out. An entire race, destroyed... because the Jedi chose to gather there. It was only then that we realized we were facing something far more powerful than we knew how to fight.-- Knights of the Old Republic II: The Sith Lords
They're saying that Nihilus was so much more powerful than anybody else they had ever fought that they had no choice but to choose exile instead of fighting him. And obviously the Jedi don't take the threat of the Sith likely.