Union of Crowns succession bug and it's historical issues.
Current patch seems to have bugged Union of Crowns. Due to that the after accepting the Union event, the heir of Hungary becomes Dauphine of Viennois (because his mother is sister of Karoly). Because of that when king of Hungary dies, Poland inherits Hungary despite Hungary still having a viable direct male heir. At first I assumed that it might be due to mods I am playing with with, but no, same happens in vanilla.
Ignoring that, in my opinion Union of Crowns while perhaps fun, is just ahistorical. The agreement stated that in the event of Casimir’s (Kazimierz) death without a male heir, the throne would pass to the son of Charles Robert (Karoly) - Louis (Lajos). But there was no plan for it to work the other way. The treaties purpose was to gain support for Casimir’s plans for Ruthenia.
The treaty concluded in Visegrád was also reaffirmed in 1350 to again garner Hungarian support in those wars (also granted Hungarians possibility to buy Ruthenia after Casimir’s death incase he has a son) but nothing I found suggests that Casimir wanted to claim Hungarian throne.
Halych-Volhynia, and Rus overall, would benefits from some kinds of situation like the mod Gathering of the Land of Rus creates. In real life, following the death of Piast prince of Halych-Volhynia, there were a nearly 50 years of warfare over that region. (https://en.wikipedia.org/wiki/Galicia%E2%80%93Volhynia_Wars).
But nothing like that happens in the game. Halych just becomes a PU under Poland without a fight. Volhynia gains ruler of Gediminids dynasty I believe and that’s it. AI also tended to lose that Halych PU, because of how it’s mechanics work. Halych is Orthodox and while you can give them Same Religious Group law through Unified Succession Law, at least in my experience, they tended to randomly turn that law back into Same Religion which meant you had to enforce the laws again, losing opinion for decades. (I admit, I have not played yet enough on a current patch to know if it was fixed, but judging by various timelapses, screen shots and other videos, I assume that it is not.)
In my opinion, region is in desperate need to situation similar to Rise of the Turks, albeit of a smaller scale. Lithuania should expands rapidly and have some sort of rivalry with Poland and Golden Horde over Halych, Volhyn and Kyiv. Kyiv should be much weaker than it is ingame – comparable to Byzantium in my opinio. Hard for AI, possible for a player. At that point in history, Kyiv was likely a tributary of Lithuania or ruled by governor of one of Rus princes from the north, but in the game, because it has good RGO near it’s capital, Dnieper river going through the whole country and no natural predator, it just gets richer, while Lithuania dies to Teutons.
I hope that sooner or later region is more developed in terms of content.
Anyway, thanks for reading.