Didn't NCR defeat BoS only through heavy use of logistics, long siegies, and mixed unit types? NCR has tanks, trucks, infantry, vertibirds and such. It's quite literally the only modern-like nation in Fallout universe as far as I remember. All of their vehicles are a bit limited, but still. They were able to basically position BoS how they needed to then siege them out.

At least as far as I remember the tidbits of lore.

Enclave is a bit different, because Enclave isn't any worse when it comes to logistics, so there is no waiting them out. And Enclave is not afraid to use old tech, to which they do have access.

Truth is I don't see how Enclave could lose. In metro tunnels, the fact they have fewer, but elite units plays to their advantage - there's a limit how many people can stand arm to arm there. In open space they are faster and tire slower than enemy, without the weakness of BoS logistics, while their armor does give them defense against anything less than a rifle. And we know Enclave also has artillery and most possibly other mechanised support so yeah...

Simply tech gap seems to big.