Hi, so we played 0.50 with YueJin for almost 3 years now (with tide extremely slowly turning to the Allies now), I guess I could add some general comments. Obviously, 0.51 is different with faster unit building times and less MPPs (10% industry for Potentia is ooof).
-When Mittelheim surrenders, all minors remain in the fight, and generate quite a lot of MPPs... but cannot do any more research. Which makes them half-useless. Not sure if this could be improved. It will be slightly better when Vikings survive a bit longer in 0.51, but only slightly.
-Guartenheim's dragon is very nice

Just wanted to state that. Allows them to fight pretty effectively.
-Heroes remain to be pretty oppressive, maybe their strength could be capped at 11

(from veterancy).
-Units will be produced much faster now... but situation with ships will probably remain bad. Maybe these are not steel monsters and could be produced bit faster? Not sure if assembling a couple of warships is so much harder then training 'elites' cavalry, heroes guys etc. It makes sea war pretty static, waiting super long for useful units.
-And, eh, 'LR bombers' are even more oppressive. On this map, with supply towns usually at 5 strength and massive chokepoints on the map, LR bombers can block any advances in many places. Which sucks/ is boring. I do not think this is easy to counter, only with air defence tech maybe, definitely not a priority. Honestly, I would just remove LR bombers :/ Not like we have Ruhr bombing campaign, and blocking advances is frustrating, not interesting play. Maybe replace them by LR aerial scouts, with 2 moves a turn and no effects other than, well, scouting?
-MPP maluses for Adversaries come pretty early. That -150 for Andania and -200 for Potentia... bites a lot. I think could easily come ~6 months later, hm.
-Finally, Andania-Angford front is really just too narrow. Towns distant from each other (so not great supply), these mountains, rivers. I understand there is some fantasy lore or whatever, but armies are pretty much stuck there, given how powerful counter-punches are in the mod. Attacker would need >150% of defenders strength. If there were two roads between countries, and/or more towns/cities, and/or less mountains, or something else making for a wider front, battles could be more interesting, not a sad stalemate.