I understand that he's an unparalleled warrior face to face, but he's significantly handicapped by his preference for melee combat. We know that it's possible for the Legion to overwhelm the NCR with hacky-slashy tactics, but a single combatant with a blade is very vulnerable to someone with a gun, which the NCR have plenty of. I don't see how he could have accumulated such a terrifying reputation on the battlefield: certainly against other primitive tribals and even the Legion, but not against civilised soldiers with advanced technology who should be able to gun him down effortlessly.