Vitiate requiring the assistance of 8k Sith Lords is only an indictment of his abilities pre-ritual. Post-ritual? It's an entirely different ballgame. Certainly he's overhyped as hell, but any credible list has to have his name on it. The argument against Nihilus is a textbook example of the No True Scotsman fallacy: the fact is that he is given a Sith title by the reigning Sith Lord, aligns with the Sith, and uses Sith techniques. The fact that he has no religious loyalty to the order itself doesn't exempt him from membership; I rather doubt most Sith Lords have a true religious faith or loyalty to the order beyond what it can do for them personally.