It works like this:
Nihilus is a serious wound in the Force, and if his hunger continues, the destruction of all -life- seems imminent. His hunger knows no bounds, and his actions of eating the Force of the entire planet of Katarr (Including the entire race of Miralukans living there in the colony and the jedi council meeting there) is considerable. He can most likely eat up a Sith Lord or anything else. Why Nihilus never ate up Sion or Kreia I'm guessing is because he was still more sane at that point or he's just not interested in individual Force users of their caliber. Ragnos, however, would shine like the sun in his force vision, and he'd try to eat him up instantly, trying to fill up the void within himself. If Ragnos can counter this, he would probably be among the select few. But from what we've seen, only someone who is himself devoid of the Force can withstand the assault. Everyone else gets pwned.
Sion, Lord of Pain, thrives on his hatred and his personal suffering. His body is fractured and pierced in over a thousand different places, and each place was kept together and mended by pure will. He cannot die, because his amazing control over his own body allows him to survive most physical trauma. Who knows? His mastery of the Force could possibly allow him to absorb ALL physical trauma, when you consider that he is the only known practitioner of this field of Force use. In any case, Sion does not die physically. He is probably barely alive in that sense. Instead, Sion loses his will to live, and this is what defeats him. Not any lightsaber cut or thrust, nor any blaster bolt or even wave of lightning. This is significant to point out because it shows that Sion has mastered his body and reinforced its fortitude with his sheer will and hatred. His power must have been immense.