I've also found that AI loves to starve towns instead of attacking them. It slows them a lot in conquering new territories. But even considering this, I find the AI good in expansionistic terms, at least the celtic factions. I've played with casse and sweboz, and in both campaigns aedui and arverni had conquered near all france before I arrive (and I arrive very soon, 265 as casse and 267 as sweboz), so I don't see problems with AI expanding, at least for now.