The game was unfinished because Lucasarts made them push the game out way ahead of schedule. They made plenty of new skills (obviously not new to Star Wars, but new to the KOTOR series), made new planets (again, not new to the franchise, but new to the KOTOR series).
Are you freaking kidding me? Obsidian made a terrible job of KOTOR2. Sotrylines unfinished, plot hole, not to mentioned the ending made no sense whatsoever. They used the same engine as KOTOR did, made no new skills, used basically the same planets, and still could not finish it.
They had a sith so powerfull he was eating force users, yet the fight against him was trivial, and wth, why didnt he just eat me?
Rubbish, KOTOr@ was rubbish.
Darth Nihilus had been weakened when he fed on the dead world that is Telos. That's why he was easy to beat. Though easy is subjective as he can be difficult depending on the strategy.
I suggest checking out the Restoration mod.
Edit: And Nihilus couldn't 'eat' the Exile because he/she is a wound in The Force, just like Nihilus.