We don't know enough about the Exile to draw a fair conclusion. Yes, she killed three Sith Lords, but how did she kill them? Did her strength lie in tactics, raw Force ability, or lightsaber skill? Did she defeat Nihilus without exploiting his bond with Visas, or did she need to in order to win? Was she nearly killed by Darth Traya at the end, or were they on equal footing? We don't and can't know any of these things.
"There is something going on in time and space, and beyond time and space, which, whether we like it or not, spells duty." -Winston Churchill
"For of all sad words of tongue or pen, The saddest are these: 'It might have been!'" -John Whittier
"Apathy is death." -Kreia