X11 is utterly broken for multi-monitor setups with different resolutions at different scales (e. g. builtin laptop screen @1.25x, external display at 1x or some variation of that). With high resolution screens (e g. 4k at only 27"), that setup is not uncommon anymore.

(Wayland is broken in very many other ways, though, so you trade one evil for... 5 others).

X was a very well designed system IMHO that could be evolved via extensions (e.g. how compositing was added etc.). It is sad that few people work on it anymore to fix such issues.

Correction: Xorg doesn't have good support for this. I don't think there's an extension for it in the protocol either, but one could be created - just as it has for Wayland. Apps that don't support the extension should be scaled by the compositor as usual.

X11 has a reasonably solid core, though, while Wayland does not.

Wayland's scaling for 1.25x and even 1.5x is not so great either, though it is slowly going in the right direction with wp-fractional-scale-v1