I think those threads are just the result of a particular battle that the player has lost with a faction with which he thought he must always win.

In fact there are no underpowered or overpowered factions. Everyone of them has low end militias, that will be cut to pieces most of the times, and high end elites, that can win against anybody. Some units' stats are debatable when the AI starts spamming them, for example the naked fanatics, that can even make the early Celts invincible, provided half of the army is composed of them.