I’ve had VLC struggle with high-res videos from my phone, particularly if I’m seeking and replaying a bit.

edit: It may have also been high frame rate.

I’ve used multiple different distros before and after, all Xorg, and never encountered similar problems. There were bugzilla reports for the Firefox issue I had, all of them attributing it to Wayland.

I used KDE with Wayland briefly a few months ago. The right-click menu would look very glitchy. Items disappeared until hovered over, and then disappeared again when moving the mouse. Firefox also had some issue where it would only take half the screen.