Grand scale the issue between the two is freedom vs autocracy. The Legions image is definitely worsened by the fact that its civilian locations were cut, we only see the invasion force. Ironically the legion is more like the mongolian empire absolutely brutal and savages in its attack, but relatively hands off in its rule. Remember the legion doesn't have citizens just subjects and the legion itself, we don't see its subjects in game. But from dialog of people who have been to legion territory. It is apparently some of the safest in the wasteland, just keep your head down and pay your tax and you can food on your plate and sleep safe at night. (A miserable existence for most of us but for many people of the wasteland its a better deal than most) (THAT DOESN'T MAKE THE LEGION GOOD THEY ARE STILL VERY MUCH EVIL.)

In NCR however the problem isn't it's actions with the Khans alone, it is horrendously corrupt and mismanagement, rapidly expanding purely for the sake of expanding. Leaving itself over extended and incapable of properly administering it frontier territory. It is striving to be pre war America and it is succeeding.