If your monitors are different DPIs then multimonitor X11 is awful.
If you're questioning why anyone would have monitors with different DPIs remember that laptops exist.
Even without considering laptops, I can already imagine quite a few circumstances where someone might want monitors of differing DPIs. I've actually thought sometimes of getting a smaller monitor I can have off to the side that I display a browser window containing mostly text on when I'm playing videogames or working in something like Blender or Aseprite; yknow, for referencing a guide, wiki, or manual or something. I don't even have a super high desire for a multi-monitor setup outside of that.
I run 2 monitors with different DPIs and X11 works without an issue. Can't say the same about wayland where scaling still has so many bugs it's just unusable.
You can't set 2 different DPIs for the monitors on X11. On one monitor everything is just going to be bigger than the other. Depending on the DPI difference it can be basically unusable.
You can configure software rescaling using xrandr and some scripts... But that can cause a massive amount of jank with anything that requires a degree of pixel accuracy
Valid point. I forgot about 4K... I run just 125% scale so it doesn't bother me at all. Well it's kinda funny that both protocols are broken in that regard.
I feel like taht's often the case but Wayland as the newer protocol usually has the correct architecture with a early implementation while X11 has hard to fix architectural problems. I am a opponend of "whatever works for you" and I think that will be Wayland for most people fairly soon if it isn't already but in case it actually isn't I wouldn't recommend it because, well, it doesn't work properly for you.