Maybe a bit of a weird question, but are there any mechanics that prevent rebel progress in AI countries, other than unrest reductions in the form of revanchism and 'national defense'?
Yesterday I tried playing as one of the Chinese Kingdom nations for their mission tree and released and played as a vasal of Ming. With Mandate at 0, stability at -3, devastation everywhere due to double bankrupt, 0 manpower and no ability no hire mercs, no rebels spawned. None. I waited for 10 years and not a single rebel stack spawned. I'm not sure why. When clicking on their neighbouring provinces I could clearly see there was unrest in their provinces, varying from 7-25.
Rebels only spawned in very low quantities after more than 10 years, but by that time Ming was being invaded by the Oirat's (with whom I made a truce of 10 years), Dai Viet and Chagatai, hurting my ability to unify China quickly.