I'm pretty far into my Austria game, for the most part Poland, former Ottoman and Russian territories will rebel quite heavily. The easiest way to repress this is to build alot of churches and make them catholic (Since they don't start off as such).

If you do not be prepared to fight rebels in warsaw, modavia, and Croatia as the years go on (Who all have slav loyalities)

As you go in the game your leader will gain unpopular traits if you keep a king. The longer you go the harder it will be to prevent rebellion.

Even when rebellion does occur it rarely has much muscle behind it. Just make sure you hit them quick, and keep all your cities decently defended.