Well the faction may appeal to historical nuts, the kind of people who want to start from the beginning.
The NCR, with all their tech and ideals, don't actually know what that entails and how logistics are important. Its like a toddler trying to build a giant death bot. While impressive, there is a lot that can go wrong.
The Van Buren documents paint the NCR in the Mojave as a shadow, a faction that grew too fast without fortifying anything. Since their hold on land was weak, they lost contact with the rest of the NCR.
The NCR in New vegas follow in spirit, but not extactly.
They follow old world ideas, but lack the world that actually made it possible. The NCR has no standardization, which is a negative for modern warfare. Lack of standardization has no bearing on melee warfare, which the legion uses, and is actually a good thing.
Without the factories, and without infrastructure those ideals fail. The NCR needs the standardization of the old world armies to compete.
They need standard training. Standard armor. Standard weapons. The NCR lacks the ability to do any of that on an actual scale. Standardization makes warfare with guns much easier, but the good stuff is held for absolute last.
Ever wonder why the first recon, the best sniper unit in the NCR, use hunting rifles that are falling apart? Which is a WWII-esque technology, when the real old world used semi auto snipers and powerful .50 cals? Because the NCR's equipment is actually limited. First recon even says its because hunting rifles are common to the farmland they pluck the soldiers from. They didn't even bother to give them anything better.
The first recon should have .50 caliber snipers if the NCR knew what the hell it as doing and followed old world mentalities.
They even use T-45d power armor as their "shock troopers." This armor was horrible, even by old world standards. It chewed through energy cells like, as one soldier put it, "chans in a rice buffet." Run out of energy, and you were trapped in your own armor. The real game changer was the T-51b, which is nowhere to be found in the NCR even though it was common in California, and they got a metric fuck ton of suits from the war with the brotherhood. California has the most military bases of any state in the Union as well, enough to field a large army. The NCR has little excuse outside of laziness to not outfit their troops beyond what they bring when they fucking enlist.
The old world nations built up their nations. Their military, culture, and infrastructure. The NCR did none of it when expanding. They would grab a toddler with a spork and spend him out to fight if they could. Armies need investment.
The NCR has little defensible infrastructure.
The NCR has general custard to World War I to 1700s era level military style. They are all over the place and it shows. The NCR lose men every day and have very few to replace them, they don't have the luxury of a meat grinder from the 1700s that kills a lot of their troops. The 1700s was a time of non standardized guns, and huge casualties but this was a time when human population was booming by untold numbers. Everything was going right for humanity at the time, but not in Fallout's world were water isn't even guaranteed. The NCR are getting way ahead of themselves. They don't have the standardization, training, entrenchment, or coordination to pull off WWI-WWII military strategies either. They sure as hell don't have any of the requirements for modern warfare tactics. Their entire military and its leadership is one big joke.
The NCR has no actual end game, or actual strategy beyond a primitive form of imperialism. Sure they get the hoover, but what next? They have water and power yet they don't even bother to entrench their position for over 4 years? What the hell is NCR thinking to NOT entrench the hell out of the dam with everything they got? The dam provides water and power for the majority of the entire west coast. That is a HUGE benefit that should be protected at all costs.
The NCR wants it all without the work the old world required. If they actually did actually open their purse and invest, the legion wouldn't stand a chance when they are sniped by master snipers with actual equipment, face shock troopers with actual power armor, bombed with the NCR's vertibird airforce, and bombarded with artillery. A little work, and NCR's problems would all go away, but instead they send the good stuff to protect a bunch of cows in California.
On the other hand, the legion and house are focused strong men governments and those always fail when the leader inevitably dies. Their progress is lost because an incredibly centralized government runs that risk when all the power lands on a single man with a cult of personality.
So its either young upstarts who have no actual idea of what made the old world ideas work in the first place, or two strong men governments that won't last a single human generation. That's what it all boils down to. The only up side is that the legion is a bit more realistic in its expectations in regards to its world. It knows its a crude government for a crude world, and doesn't try to bullshit you into thinking its America 2.0.