The whole problem seems to me that CA contrived it so that the Romans would do very well, whether controlled by the AI or the player.

In doing this they had to ensure that the Romans won a large proportion of autocalced battles, and to achieve this the Roman units' stats were souped up.

Also Celtic, Germanic and other "barbarian" warriors were fearsome fighters and should excel in the "bumrush", whereas the Romans should excel in using cautious advance, and strong tactical formations, relying more on their morale and guts.

Again the problem is down to weak AI. Unable/unwilling to find any other way to represent the superior Roman tactics, deployment and logistics the developer has simply treated the Roman units in RTW like the Saxon/Viking Huscarles in the MTW/VI campaign with artificially increased stats. In this campaign, Anglo-Saxon hegemony was assured by the presence of these uber units in the unit roster to imbalance autocalc battles in the Anglo-Saxon faction's favour.