Every single legionaire has the same base armor.
Game limitations in my view, since I don't see how they could all have the same matching football uniforms and each have all the same equipment (same goes for the centurions who take armor from their fallen foes). Some may have more armor, some may have much less.
I won't split hairs on you with that though, since you're right in that its not stated in-game.
In my view, where the Legion has an advantage is in unwavering zealotry and discipline. The NCR
wouldn't be able to send soldiers in armed with Melee weapons in a "mass human wave" and expect their charge to hold (in other words their soldiers die instead of running away), the Legion can.
I guess what I'm trying to say is that the Legion's tactics are perfected for dealing with other tribal societies which they have been fighting in the east. Apparently, in-game lore they can stand up to the NCR (although I contend that a good part of this is due to the NCR having been unable to mobilize its full might). But I think its a stretch to claim that realistically the Legion's tactics would be viable in this same situation in real life. That's my point I suppose.
Add in the complete lack of modern medicine, medics, etc and you lose a troops that you shouldnt be losing in the first place.
This is true as well. The Legion's battlefield medical techniques are (by virtue of their culture) extremely limited. I'd imagine that Legion battlefield casualties have a much less survival rate than those on the NCR side.