The problem is that a centralized body immediately becomes too powerful. The body itself would need to be limited in scope with other outside checks and balances.
SCOTUS is close to one of these centralized bodies you speak of, and they purposely only deal with cases that have gone through every other avenue. And even then, they are not trying to determine truth, but only fit the verdict within the existing body of law. I would prefer less decisions being decided in this manner.
I prefer giving power no secrecy to hide behind and limiting it by democratic control. I dont know why the assumption is always that a central power has to be some opaque inscrutable beurocracy.
I don't know why the assumption is always that a democracy is incapable of tyranny. Gay marriage had been illegal for most of our modern democracies' histories under the tyranny of democracies. Prostitution and drug usage are currently illegal under the tyranny of democracies. In England the manner in which you discuss large scale immigration or transgender issues, or what pornography is acceptable to watch is subject to review under the tyranny of democracy.
SCOTUS is close to one of these centralized bodies you speak of, and they purposely only deal with cases that have gone through every other avenue. And even then, they are not trying to determine truth, but only fit the verdict within the existing body of law. I would prefer less decisions being decided in this manner.