Without JCW Nihilus wold've been hunted down by mere army, droids and such. However, chaos and destruction allowed Nihilus to slip from one shadow to another and strike from there.
Besides, anyone who can cut themselves from the force for some time can take Nihilus in one to one combat (that Luke's weird technique). If they are capable enouhg otherwise. His "strenght" (it's nothing but a curse) is feeding on the force. It is ancient sith technique. However, there must be other ancient counter technique. Otherwise the one(s) who first found that technique would've just eaten everybody like Nihilus did, ending the world. Poison is eradicated with poison. Attack is stopped by counter-attack.
I believe TS knows this (as they may know Nihilus's technique) counterweapon and Revan too as he now is with them. And we don't know all the ancient knowledge Revan learned at Unknown World and Malachor V. Thus it is quite likely that Revan (and any other that would gain access to the info) could fight against creature like Nihilus.
That is my personal thinking since there is no K3.
Yet Nihilus's power is not true power. He is nothing but mad, raving beast, obsessed with his hunger.
Anyone with quick wit could bring Sion down (or to be more specific: his morale and will to live).
Exile was nothing but average jedi until Malachor V (and even after that it was not truly his "own" power, just something leeched from other persons to be annoyingly precise). His/her "unique" strengt is capability to form bonds with ease.
"The real difference between the Exile and Revan and/or Malak, is that his authors took the time to give a reason for his above average or exceptional strength in force, rather than simply say he was a prodegy which would have been less interesting."
Apparently most jedis and siths are extremely powerful just because they are. They don't need goofy bonding abilities
Sorry for being negative, I love and hate at the same time idea of Nihilus and whole "eating up the force" thing.