The tribal tech is a real problem. I think a neighbor bonus should be added for Tribals. If they are located next to advanced nations, they get an increased research bonus like if "Behind Times". If the advanced nation is a subject, the tribal gets another boost to research via "Sharing". This would encourage tribals to vassalize city states instead of conquering them (which had historical pretext).
Imo the problem is not the tech (or only marginal), is pops, and here is where Rome's buffs are insane, even if is not direct, but a consequence. In regards to pop growth, except some super-minmax and surgically choosing a few territories ingame, you're looking at a pop growth rate of 0.25 to 0.4 or so. That means one extra pop per territory in 20ish to 30ish years. If in this time Rome is getting for free the whole Italy, is also getting this first wave of pop growth, which is an exponential growth for the tag itself. And then it only snowballs from there. If afterwards in 30 years it goes from 3k something pops to 7k, is irrelevant if the new 4k are wrong culture and wrong religion (because as seen above this has no effect on Rome). And after other 30 years or so, they will be right culture and religion pops, with even more newly generated pops, with their levies, manpower and income just snowballing.
The devs purposedly balanced pop growth in 2.0 exactly to not have this snowballing effect, but they let it be for a few selected AIs, with the other hand, via expansion.
Hence to counter this, or rather to stand a chance when the time comes, the only viable strat is to gain territories extremely fast, so even with a modest tribal growth you'd stand a chance to create a reasonable pool, which later to be specialized with cities. Which means colonizing as a migrant tribe, due to the forced culturalization and conversion of the settlers. And not only to get territories and pop for yourself, but also to block Rome. So you need to cement somewhere either in Illyria or close to it, speaking from an eastern perspective, respectively +- Provence from a western one. Surely you can also conquer, but again as a migrant, and rather in a limited manner, since you need to somehow contain all the AE, WE etc.
And is simply a bad mechanic if migratory is the only choice for all tribals bordering Rome or being in close range, and it has zero connection with how actual history went. Neither Gauls, neither Dacians, neither Germanic tribes were a pushover for Rome and this is not reflected at all ingame. And is making the whole centralization path redundant as a choice of gameplay, its only merit is to be an appendix for migratory gameplay.
They really need to have those claims toned down, AI's stability needs to be toned down, there must be revolts and AI must be able to take in account how many provinces would be able to control afterwards. There need to exist some sort of coring costs, some budgeting of all these actions, otherwise as it stands now, AI Rome (and Egypt and Maurya) are not actually playing themselves I:R, but some sort of point and click.
If some players need more challenge, then just make a hardcore setting, which to combine the AI buffs to stability etc with whatever "hard" brings and make it a choice at the start of the game. But as it stands now, with AI free to expand at literally no cost, this is not "normal" level.
