It is more of an observation, especially in my current game, where I have almost forbidden all barbarians to occupy any assets at De AI Arbitris. The Arverni did not care, nor did the Lusitani, Arevaci and Ardiaei. They conquered it cheerfully. Also Bythinia was suddenly in a corner where I did not suspect.
These are the ones that I have seen and all are original fractions that are playable and certainly have some guidelines as well as Rome. And I'm afraid that they try to push them through. So I think it would have to be deposited somewhere.
I hope someone else can confirm this as well. Most of the factions gain ground, which are playable in advance in the game.