Countries should not be allowed to take provinces which are only adjacent to vassals or PU partners (i.e. vassal and PU partner provinces should not count as adjacent for coring purposes). The problem with this happening in-game is that it results in unrealistic border gore.
You are right. PU are pretty badly represented in game, as a type of vassalage. In reality, it was more like an alliance of nations (with their own parliament, laws and customs) sharing only a common monarch.
Thus, Lithuanian claims on crimea or Swedish claims on Karelia would, could and should never have fallen into the hands of Poland or Denmark. This would have simply been angering local nobility or powers for stealing their claims
Finally, it doesn’t make any sense gameplay-wise, since if the junior gains independence (hey there Sweden every game), then the senior partner finds itself cut from its provinces (thus unable to defend them, reduce unrest or fight rebels, unless he already has a garrisoned army there)
In fact, a very good example of the bad representation of PUs in game is that Lithuania never came to war with the Teutons in 1466 ; and likewise polish nobles often rebuked at paying costly wars to defend Lithuanian territories. This in it’s time led to the Union of Lublin, when the polish nobility asked this as a counterpart to involving into muscovite-Lithuanian wars
