Some peoples resisted for centuries after annexation, some countries always had to deal with various minorities and take them into consideration, and some nations simply had very hard time managing conquered territories due to local resistance. Some peasant revolts led to full-fledged civil wars with rebels often victorious, some religious fanatics kept their rulers of other religion in constant stress. For some reason the game doesn't care to actually transfer the feeling of struggle against rebels. You almost don't have to manage your conquests, just conquer something, wait for unrest to go high enough to spawn some rebels, kill them and be done with it. But that's not how it works. Some nations never manage to emerge because rebels are simply too simple (unless it's special mechanics like the Netherlands). You never ever see Greeks breaking free, then working hard on establishing themselves as a state with international relations and kick the Ottomans in the butt. You never see East Slavic rebels returning their land from Lithuanians, even though they made up the almost absolute majority of the population and military of Lithuania. Rebels are simply a pain in the arse at the moment, nothing dangerous about them. You almost don't need to manage your conquered areas, you don't need to deal with stubborn populations. After having gained about 20 provinces, the game turns into a piece of cake.
