I find that Prussia tends to destabilise the initial phase of the game with its invasion of southern Germany. They tend to cut up Bavaria and Wurttemburg, thus cutting French supplies as they go into Austria. Frankly, I'm not sure that Prussia at that point in time had any ambition in that direction, but was rather fixated with Russia and Northern Germany. Its also ironic that it attacks a French ally which forces the French to either ignore Bavaria and loose its supply line to for its invasion of Austria or else declare war on Prussia way too early. Is it possible to redirect Prussia's attention elsewhere?
Another thing that I find really weird is that the coalition tends to spread it forces in penny packets all over S Germany, making them vulnerable to French attack.