Pretty much the problem. We do not see the Legion beyond the frontlines and the atrocities they commit against their "enemies."
If we actually were able to cross the river like in that Arizona mod and actually see what life is like under the Legion's rule they'd actually have some merit beyond just the "Faction I pick purely to piss people off."
We hear about how safe the roads are especially for traders under the Legion but yeah, we never fucking see that shit.
If I were to imagine it, I'd imagine it would probably be rather authoritarian, but peaceful. You stay in line? We all get along, but the people who do dare to step out of line are made an example of which scares others into rethinking doing the same.
At least that'd be my best guess. Peaceful, but only if you follow the rules. Which would make sense given the state of the Wasteland you would have to use some kind of force to make people behave after all.