I think in a narrative sense, in particular in New Vegas, NCR is the safe bet - in the sense that you know what you get, for good or for worse. With any of the other, there's uncertainty in their lofty promises. Even if NCR is kind of lackluster in their ambitions to function properly, it's a post-apo gvt transition that seems the safest available option for the ordinary folks
Well, there is also the fact that both in the real Roman Empire, as well as hinted-to in the New Vegas Legion, there'd be copious ammounts of buttfuckery, butchy men taking soft men as lovers, shaz like that. As much as I'm up for sexual freedom, having a whole society based on hierarchical gay sex just ain't my most fav cup of tea