I dont think any unit is overpowered.
I fought elephants ...tough but i managed.
Urban cohorts need 2 turns to build and thier stats arent much better then legionary, they are more expensive too.
If you got a army of urbans...thats a lot of time and money...

Wardogs are awesome vs. unarmoured poor gauls. But try them on hvy infantry aka greek phalanx and u see how good they really are...
Gaul warbands...I dont think I know a weaker unit in game beside peasants..specialy without a general and the AI often comes without one.