If anything, I'd find it less believable that the NCR's terrified of a band of barely-organized strung-out hippies who're only stopping themselves from killing one another so long as someone keeps their supply going than the biggest, most organized faction the NCR's ever faced since the Brotherhood of Steel.
People who keep crying about how the Legion shouldn't be winning because 'LOL FOOTBALL GEAR AND SHARP STICKS' keep forgetting that the Legion's been spending the past five years since their loss at the hoover dam basically shattering the morale at the NCR without resorting to outright battles. Hit-and-Run attacks on caravans and patrols, radiation bombs on NCR camps (and if Vulpes could get away with it, chlorine bombing the strip), raids on outposts and settlements where they crucify everyone they haven't raped and sold to slavery. Hell, by the time you arrive in the Mojave they've just started probing further inwards to see how much they can get away with. The completely green NCR troopers, due to a combination of general incompetence, glory mongering, and bureaucratic interference, are essentially stuck fighting trench warfare against an enemy that not only heavily outnumbers them, but have been indoctrinated to be fanatically devoted to their leader and are really good at working wonders with those machetes they're carrying. About the only silly thing about them are the costumes, which are, once again, just football padding, which works as fairly cheap, easy to produce armor which is effective when you're trying to equip an army in the thousands.
