Theoretically, Wayland has proper support for fractional scaling, and this just works. In practice however, this only works for wayland-native applications and all applications running through XWayland will be rendered small and scaled up, which ends up looking terrible. I guess this is implementation dependent, but this is consistent across Wayland on GNOME, Sway and KDE Plasma.

This is unfortunate, since many applications can scale just fine, but only through older hacks. Chrome and Electron applications will scale just fine by providing a proper DPI. Same for very old apps. GTK and QT applications will not, but they scale just fine by setting appropriate environment variables.

In practice, this leads to the experience often being better on X11. I've found KDE X11 to handle this the best, with the limitation being a single global scale, only certain fractions allowed and needing to restart KDE after changing it.

GNOME with Ubuntu patches and Xfce will scale everything x2 and use RandR to provide a higher virtual resolution. This has performance costs and messes with cursor speed (measured in pixels) but otherwise also produces good results.