I witnessed one situation where Ming (China) lost the Mandate of Heaven (a +5 Revolt Risk condition), had -3 Stability, and suffered a series of usurpers to the throne (low Legitimacy). Their annual tax revenues were reduced to only a smidgeon above 0, to the point where they couldn't afford to field 10 brigades of troops to put down the multiple 12-14 brigade rebel stacks. The rebels repeatedly took over the government, resulting in yet another illegitimate ruler who fails to pay his bills, or another economic collapse, and permanent -3 Stability. Of course, I eventually had to step in and save them by taking it over myself.
I've also seen France wracked by a series of large revolts, with 75% of their provinces occupied by rebels and pretenders to the throne. Once again, it presented a golden opportunity, and a century later, the trivia question of the day was: "Where was France?".
The AI may get a few breaks, but it's still heavily affected by Stability, Revolt Risk, and Legitimacy issues. These can be used to your advantage, if the circumstances turn out right. Throw in a bit of War Exhaustion, and it rapidly turns into a toxic mix that the AI will choke on, although not always when you need them to. I've seen enemy countries all-but collapse a few months AFTER the end of the war against them, and then again, sometimes those revolts will erupt annoyingly all over the country after you've broken their army and are trying to siege, and then you have to fight the rebels to finish the war. I had one game where the powerful Ottoman Empire declared war on me, and abruptly broke into scattered revolts which drew off most of their army, and then I was able to successfully take on their returning forces in bits and pieces, turning almost certain defeat into a relatively easy win.
Incidentally, I ALWAYS turn off "Lucky Nations".
