All of the gameplay mechanics surrounding culture & cores are unintuitive and not fun.
All of the information regarding culture, culture groups, and languages are buried across a variety of different map overlays, diplomacy interactions and just vague and unclear game mechanics that make engaging with those mechanics an overall unpleasant experience.
I think the Vassal meta was a simple solution for players to just reduce what the player has to interact with via cultures to a very simple loop - Make small vassals everywhere and just have them enforce culture & religion until you can annex them as a core.
Now much more care needs to be put into how you create subjects, where you create them, farming your subjects off to other subjects, and gaming the system to eek out as much positive % conversion rates, such as making your primary culture consider the ones you want to convert an enemy, and making the cultures you want to convert look at you positively. The player can fully waste their cabinet action for years trying to convert culture and the game will never give you any indication of this without you watching it & all of it's contributing factors.
I dunno. I agree that there needed to be some design adjustments to stop how easy it was to snowball, but between the vassal & culture changes in this patch, and with complacency from the previous major patch, I find myself more and more frustrated with and turned off from the direction the development team wants to take EU5 in.