The problem I see is that guerrilla warfare works best when the enemy isn't out to conquer your land. As for demoralizing, if NCR can convince it's people that it's victory or death in a war against the Legion, then the demoralizing effect of guerrilla warfare might be reduced.
There is also the chance that people in Legion land that NCR takes might see that as a good thing and side with NCR. Whole armies of slaves the NCR could get to rebel. Like with the Roman Empire. It did a good job keeping the tribes it conquered under foot, until large outside forces got them to rebel against Rome.
Then again every legion town could fight to the death like Japan threatened to do. It all depends on how fanatical they are and how much they like Caesar.
Actually Guerrilla warfare is the single most effective way to bring down a world power no matter how big your military may be. There's a reason why we ended up losing the non-organized battles of Vietnam and Afghanistan. When your enemy doesn't play by the rules, uses quick small strikes against small military targets, and above all doesn't have a distinct uniform. (Something which the Legion fits minus the lack of uniforms since ya' know..they wear football pads and all). Its very hard to win a war without taking sever causalities both in the literal and political sense.
That said I do think the NCR would win eventually, but it would be political suicide for whoever lead the war for however long it lasted.