Power corrupts, and absolute power is a lot of fun. But considering how many weapons the Concilium Conventions have stopped me from fielding (0) I find O-12 difficult to take seriously. Perhaps the Aveng- I mean, Optimates (sounds like a line of carnal recreation dolls) will change my mind.
Restrictions in the name of human rights usually only apply to factions that are policed by the enforcers, which the enforcers are typically not.
The O-12 are pretty legitimate as far as international organisations go. They do their best to stop the Sphere tearing itself apart and can legitimately point to various existential threats to justify their powers and intervention. I also get the impression they're relatively optimistic, ethical and hard working, I think gutier mentioned once that people working for the O-12 are overworked and underpaid, you do it out of genuine belief.