Why does everyone hates barbarian factions?

[Vincent Butler]

The Bull Warriors are possibly the best infantry unit in the game

I think they gave them for balancing purposes. I don't know that I would rank them among the best, though I don't remember their stats. The best is probably Spartan Hoplites (insane attack, both primary and secondary, and high defense). But by the time of Rome Total War, even the Spartans weren't anything special compared to other hoplites. Bull Warriors absolutely should not have two hit points. Again, probably a balancing thing. I would imagine that the primary barbarian weapon was a spear, so the barbarians should have more spear units than just Warband. Then again, that is probably a decent representation of the standard barbarian unit. They had good swordsmen too, but the Warband would probably make up a vast majority of a barbarian army.