1. X can't handle independent screen scaling of two different DPIs. I have 24" and 27" screens and on windows I can have the 24" one at 2x and the 27" one at 1.5x and it "just works". In X I have both at 1.5x.
2. Wayland supports independently scaling the two screens. Unfortunately programs that don't support wayland run through something called XWayland, which upscales them in a crude way that makes the fonts blurry. It's pretty much unusable because of this to have an XWayland app (this includes chrome, firefox, and emacs) running on a 4k screen in wayland, at least on KDE.
It's frustrating because it's SO CLOSE. There are WIP branches of both browsers running on wayland (but they're not stable for me) and once those land I think everything will finally work.
All this said I do still use KDE on X daily as my development environment, but every time I boot back into Windows I'm frustrated again by how much better the scaling is.