Wasn't every one agreeing on how OP Prussian armies were? Similarly, I've never seen a successfull Venice; won't even start on Genoa which always get crushed by Crimea/OE, Aragon and Milan.
As for Russia, it's OP because it has free room to crush week hordes and Asian while colonizing 5 BT gold provinces with no one to stop them (they are bordered by PU minors, OPM left to be annexed, poor Novgorodian punchbag and a couple hordes and on top of that, they get missions with claims on all of that just to make sure it's not too hard to core it all). Other nations do colonize, but with long traveling time, overseas penalties, and some mere 1 BT 5 manpower province with 10k agressive natives.
Yes, Prussian armies were OP, but then, you begin with a small country and AE inside the HRE is pretty high, forcing you to go against Pomerania, TO and Poland. And, by the way, Japan's also had an OP army, but that's the point, isn't it? It is a poor country, but can stand against giants with its fierce army. Of course, there are a lot of cheese gamey ways to overcome this problems that Brandenburg and others face, but in the end of the day, Brandenburg/Prussia economy is basically tax-based, has little chance to outcompete other nations at the relevant trade nodes and so on. I'd say that having a better army than the rest was it's charm, in a lesser degree same thing goes with Sweden... they simply have to do the best army as possible with their small resources. When you close the gap between a militaristic nation's army and a trade/economic nation's army, you are clearly creating a situation where trade/economic are clearly better, as they now have even more resources. And well, I cannot even remember when Brandenburg survived when played by the AI in most of my games.
I'm just saying that... maybe the Discipline nerf was a bit too much. +10% Discipline would be better, there was no need to nerf beyond that, as most military related bonus are being nerfed while economic/trade bonus are being amp'ed.