Most of the lower ranks are very badly armed and armored. Historically speaking, what happens when you get a bunch of guys with melee weapons to charge guys with firearms? A lot of dead natives during european colonial conquests. Sure, NCR troopers aren't exactly like US army rangers, but the service rifle is still a 5.56mm semi automatic rifle. Each NCR trooper should be able to take on several legion recruits (guys with machetes) easily in most conditions. Obviously that would be quite imbalanced game wise, but lore wise, it seems that the bulk of Caesar's army should be highy ineffective except during night raids, etc. Even the assault at hoover dam as potrayed in the game makes little sense. The topside of the dam is largely an open killing field with several towers, and the NCR has plenty of snipers and machine guns. And the assault was launched in broad daylight to boot (cant remember if the time depends on the player's actions). And swimming up the pipes (or whatever it is the legion does to get inside the dam) seems highly implausible.