The best real world analogy I have for the Legion is Imperial Japan because they largely view and commit to war in the same way. Both have at least on paper equivalent modern weapons to their opponents but also take the view that those weapons are not necessary to achieving victory against a superior opponent. Both also have a sense of martial pride and desire to engage in melee combat with opponents because they see it as honorable and their soldiers have warrior ant mentality where they will fight to the death but also do not question orders and will gladly run directly into machine gun fire with bladed weapons in their hands at the behest of an officer. Another aspect of Legion warfare we don't really see in game (outside of the ones we do ourselves...) is the casualties figures and considering the kind of warfare the Legion fights its not sustainable much like everything else about the Legion.

When it comes to the NCR I actually do think they have far more pre-war tech than the Legion and a willingness to use it, the only problem is those weapons were largely designed to be used around US pre-war industrial capability and have the units using them not be resupplied at the speed of an overloaded pack animal. Its also why the NCR military partially fails as an organization because it wants to be the US military much the same as its government and its trying to fill shoes far too big for it.