I always find Poland seems to do well at first but then stall once it runs out of rebel settlements to capture. It is one of the few factions with lots of nearby rebel settlements to grab and no initial threats until it has expanded enough in any direction to meet another faction, but the settlements near it are pretty worthless. This is what I found in my Polish campaign; I very quickly had a huge empire of worthless settlements.

I've never seen France do very well, nor Hungary. I suppose the way the game works guarantees that as soon as they start to do well against one enemy they will be backstabbed by another.

I've actually seen the Byzantines get quite powerful and conquer most of the Turkish provinces, in a game where the Turks and Egyptians got bogged down fighting one another. Those three factions seem quite well balanced to me, the outcome in that region seems quite unpredictable. A pity it is all rendered completely irrelevant by the inevitable Mongol steamroller.