Seems once you can recruit stacks of these units, you are unbeatable to non-roman factions. The severe imbalance is a little irritating and makes battles hardly competitive. I am finding that 3 units heavy onagers, 7 units praetorian cohorts and three units urban cohorts is invincible. Just bomb the enemy army until all of their units are well under stregnth and then tramp them into the ground with the cohorts.

History has praetorian cohorts as imperial gaurds and generals body guards only.
For the sake of balance in the game, if not historical accuracy, it seems such super units should be pretty rare. Given that these are "picked men", it would be alot more balanced if you could only have one praetorian cohort recruited with every 5-10 legionary cohorts in the field. Likewise one urban cohort (forget history with them) for every 5-10 praetorians. The uber units being composed of the best men of existing lesser units.