They could easily balance the relative strength of the units by how much they cost in denarii. Unit cost is abstract and is needed for gameplay purposes but didn't exist in real life like how it exists in games. CA could easily use them to balance and at the same time keep the relative strengths of the units toward each other as realistic as possible. They could keep the legions as overpowering as they were in real life. You just can't have as much of them as you could other weaker units using other factions.

In single player, however, the factions don't really need to be balanced. Some historically powerful factions should start with more developed and richer provinces and have access to more resources.