But he never did that, now did he...
He if Dark Side stole the power from the Masters, well three of them. But not from Kreia who easily slaughtered those Masters and could have defeated all them with ease even without her insta kill technique. The Exile also doesn't absorb Atris her power, or Nihilus or Sion. and thats the problem once he takes Nihilus he becomes whole again, but he doesn't get Nihilus his power... He's just himself again.
He still for some reason has his borrowed powers, perhaps it was for gameplay reasons or perhaps just because he should have kept them, but he won't absorb power anymore. If Dark Side he could still be very powerful.
If Lightside he would only be as powerful as he would have drawn at that moment, which simply can not compare to Revan or Yoda for that matter. The Exile if lightside has to do things alone and is simply said, screwed. His fast learning days are over as are his rapid improvements. and then he is again what he has always been, a mediocere Jedi this time however with a bit more power. But he simply isn't good enough to take Yoda or Revan.