All systems of leadership have corruption and tyranny, but it's a hundred percent more likely when your local area is run by a single man (abolute power corrupts absolutely) or a group of gangs who were tribals just a few years ago.
The NCR is a constitutional Republic, so corruption can be rooted out through voting and leaders do not have abolute power. The leaders may not be the best, corruption exists and they're not free of all sin. But it's a better system, and ensures a higher quality of life for people inside it.
Just because the NCR is better though doesn't mean it's better for the Mojave. And to be honest, I don't think the NCR is better off with the Mojave, they're overextended as it is and taxes go higher and higher the more they expand. They need to chill their tits, focus more on the industrializing the lands they have than civilizing the lands they don't and come back when they have more to offer.