This is the clue!
When I attack enemy, there is so much time that troops from the other side of country can join. It's completely unreal and ahistorical and ends with all the forces from both sides concentrated in one only province and battle of mega-stacks. That means less army micro-management, because I almost always have time to help my troops that are somewhere else. I have no possibility to crush one enemy to take care of another. If I try to do so, I will fight with both simultanously.
This is even more annoying when fighting on seas. Every bigger naval battle end like this - the whole my fleet vs. the whole fleet of rival and the one who loses, loses the war on sea (it's almost always impossible to rebuild fleet after such a battle before war ends)
I know - it's hard to invent better system. But I believe it's possible. It was possible to improve the fortresses system (that was as old as EU serie), it was possible to change traditional trade system and get rid of static historical events from EU2 (I couldn't imagine the game without them those times). I thing it is also possible.
I thing the troops in one province shouldn't be obliged to fight. There could be percentage probability of battle modified by offensive or defensive attidute of units or some other mechanism. I see some possibilities to make things better.