I'm running into problems with barbarians. Not because there's too many, but because there are two horrifically bad bugs.
1) Barbarian numbers do NOT reduce when I fight them. Their morale does, but their numbers don't. Mine do, however, which means that all I can do is hope that I'm strong enough to beat them down until their morale is so low they get wiped.
2) When the AI is at war with me, barbarians become non-hostile to them. I had 20k barbs help my enemy siege down a fort.
Combine those two, and you get the occasional situation where an enemy army becomes unbeatable except by massively outnumbering them, because I have to fight a combined enemy-indestructible barbarian army.
Imperator: Rome
EDIT: Well, the game's now actively fucking with me.
Who would win: An army of light infantry, light cavalry, and chariots that outnumbers me 1.5:1 with a commander 1 better than mine, or an army consisting of chariots, heavy cavalry and heavy infantry with 15% higher discipline, better military tech, that is defending in hills over a river crossing with another 10k troops of similar composition on the way which will invert the outnumbering?
If you guessed me, you'd be mind-bogglingly incorrect. My army got *stomped*. I then hired mercenaries. They got to full morale... And the game treated them as if they had 0 when they got attacked, resulting in me getting stackwiped the instant they were attacked. And I mean instant. The same day that the enemy army arrived in the province, they apparently all decided suicide was better than fighting.