No, I have the experience of letting my friendship with the neighbouring Mongols lapse and next thing I see is a carpet of their cavalry armies rolling over my countryside, supported by field cannon armies and an occasional bombard corps or army, plus mixed in levied units. And they take my cities in one-two turns. Although, it seems, AI is still reluctant or not focused enough or just not programmed to go for targeted and complete elimination of other AIs or the player, and it lets the taken cities rebel and flip back to the original owner.
Maybe try a few games with no mods, to get the impression?
If the AI is stifled for space, they stagnate and fall back. But if they manage to blob... I'm facing Mongolian GDRs now at t300 on deity, like never before, my coastline is being swarmed and pillaged to oblivion by their nuke submarines, and he nuked my buddy Harald three times already. Nukes targeted the same city three turns in a row, so that's still not ideal, and after capture that city flipped back, so there's still a lot of room for AI improvements. But AI definitely got tweaked for better in some aspects after the September update.