@Brokatt: Heya, thanks for clarifying it a bit. That way the possible contradiction makes sense. Still gotta find out if I can keep using X11 with Kubuntu after the update though.

As for the problems, it was all kind of different things. A very good example was the last odyssey of trying to get certain games to run properly in 16:9 on my 32:9 screen. Many new-ish games (for example the System Shock Remake and Nobody wants to Die) behaved very strange with the resolution settings. Obvious solution for me was to run them via Gamescope in my preferred resolution. But in Fullscreen and Windowed-Fullscreen modes, Gamescope always kept the game on the left half of the screen (I want them centered). I tried a lot of possible solutions with forced positioning until I found out unlike X11, Wayland seems to just ignore them. I then tried to solve it with the Window-Behaviour-Tools (awesome tools btw.. as a Windows native from 3.11 to Win10, I could only dream of something like that), but could only get to a halfway acceptable workaround... basicly making windowed Gamescope look like borderless, but I either had to switch my taskbar to auto-hide or make the window always-on-top, so Alt+Tabbing wasnt possible anymore... both stuff I´d hate.
After a while I simply tried X11 just to notice that I don´t even need gamescope anymore because all the games where behaving just as I wanted while also solving most other problems I still had on my todo list and make most of the workarounds for problems I already solved unnecessary.

I can imagine most of the problems I encountered stem from me using a 32:9 screen and a rather new GPU (RX9070) and there are probably better solutions than the ones I worked out, but for me it was over a month of problem-whack-a-mole with solutions often leading to new problems that I could've spared myself by just using X11 from the start. I believe Wayland will be the future and it will be a good one, but for now I really want to stick with X11 until Wayland works better for my setup and preferences.

Since you asked, some other problems I encountered off the top of my head (I had solved some of them and for the other I was glad I no longer have to bother):
- Search within in the Steam-client (STRG+F) didn´t work properly (seemed to be some window-focus problem).
- Videos from Firefox's picture-in-picture mode didn´t stay on top
- Everything in the app "KolourPaint" looked ugly/oversharp.
- Some rare seconds-long graphics bugs with wrong colors, flipped image etc. that seemed more like typical effects of a damaged-cable or port (completely gone with X11, but they're back when I switch to Wayland).
- Like in my example above, I encountered other possible solutions for some problems that would've worked with X11 only to later find out the problem itself doesn´t even exist in X11, so the solution would work but isn´t needed anymore.
- I still have some wonkyness here and there with focus-behaviour of some windows/apps that I have to hunt down, but it feels like its down to only a third of what it was before. This is very subjective and might have to do with other stuff or just my imagination or some changes in my own usage behaviour, not sure.