To a limited extent, that is true. But you also need to ensure loyalty of the troops. Money can get you only so far. Even if I am paid 10 times what I am worth that does not mean I am unable to plot against these strange people. In fact the money they gave may be quite useful to stage a rebellion. The bigger the cultural differences the less likely it is you can ensure enough loyalty for the troops to be dependable.

The fact that Roman legions did not serve among the people they were raised from (if the legion was raised in Gaul, it would not serve there) illustrates the point.

I have no issue with recruitment black holes per se. But there is an interesting related question.

The Romans start with limited recruitment, but by the time Marians hit, they can recruit Marian troops all the way up to Seleukeia. Thus the succesful expansion and military reforms lead to an increase in military recruitment zone for factional troops. This does not hold true for other factions. I am well aware of the historical nature of this.

I think the biggest issue is when factions expand beyond what they historically achieved (the smaller factions come to mind). Then recruitment can be quite limited.

So you may end up being forced to ship in the garrisons to keep control of these settlements, even if you completely reshape the settlement as a city of the Getai / Lusotannan / Hai etc..

If the Hai would have recreated the Persian Empire (plus northern Aigyptos) and would have kept it stable for say 100 or 200 years, undoubtedly areas of recruitment would be completely different, as would the military units themselves. Reforms would / could have kicked in, because of economic reasons, related to combat styles of the enemies etc..

This is no critique of the excellent work the EB team has done. Most of these are simply due to engine limitations. Or we would have to depend on speculation on how the Lusotannan military would have dealt with the Saka riders. Who knows? But EB can't include units in the game, based on pure speculation.

I don't think it is possible for the game to track how long a settlement has been continuously owned by a faction.

It would be lovely if it were possible to build lvl2 governments in "outlying" areas which have been thoroughly been "Getainised / Lusonised" etc. if a long and complex set of requirements have been met (similar to the Hai reforms), with an increase in available factional units available once the MIC has been built up.