Well, it is a tad more complicated than just that. Troop deployment would be about the same, which is worse than Civil War times.
The North had all the advantages you listed, but got their butts kicked up until Gettysburg. Huge advantage for the North was a much more extensive railroad system than in the south, which helped with deployment, supplies, etc. This does not exist in the FO world.
A lot of the NCR military are green troops. Legion is fairly battle tested. In situation like this, just about anything can happen.
What's your point about the Civil War? The real-life Union was a lot more advanced than the NCR, but the Legion is far beneath the Confederacy, so if anything, the NCR has a greater industrial advantage than the RL Union.
You do realize that in most western armies, it's not the fighting man, but the noncommissioned officers that are the backbone of the organization. Enlisted personell come and go, but it's the noncoms and the officers that hold it all together. And since they've been at war with much stronger foes than the Legion in recent decades, I doubt most of them are "green".
Well, if Legion wins in Mojave, there is a Gun Runners facility. Make us weapons, or die. You collect the weapons and ammo from your enemies. You raid enemy production centers. It isn't like the Legion wouldn't be able to get spies or covert ops guys into NCR where a few well placed explosives, say at a weapons manufacturing plant, would cause a lot of damage.
If you think that is all that matters, then maybe you can explain Russia in Afghanistan, or the Vietnam Conflict.
Scavenging weapons and coercing a small outpost of a company that for some reason hasn't fled with the rest of the NCR *if* the Legion wins in the Mojave is your grand strategy to make up for the Legion's nonexistant industrial capacity? That dosen't even warrant a further response.
There is a big, big difference between a conventional war and a guerilla one ><, and a full-scale war with the NCR would fall under the category of the former. If you want an apt example of a industrial army and a preindustrial one fighting a conentional, pitched battle you should look up the Battle of Omdurman.