If units are priced according to their strengths then in Multi you can afford to add enough valor and armor and weapons upgrades to equal things out when playing some of the weaker factions.

But in the Campaign, I much prefer to have some really strong and easy to play factions (ahhh, the Vikings!!!) for those days when you just want to kill things, and some really weak, hard to play factions for when you are looking for a serious challenge. I think RTW has done that really well.