BTW, Windows 10 is slightly better, but not by much. The major difference being that one can predictably get a signal on both monitors, I guess, which is good, but then the HiDPI story is still pretty bad.
To name a simple example, pressing CMD + Shift + -> moves an application to the next monitor, with macOS you should pay for a window manager to get a similar functionality, or give up the keyboard and embrace a touchpad-centric gimmick like Expose, which I perceive as distracting with these animations.
It is certainly an example of YMMV.
Meanwhile, multi-monitor support in Plasma 5 has worked for me for years without any trouble whatsoever.
* https://bugs.kde.org/show_bug.cgi?id=365455
* https://blog.martin-graesslin.com/blog/2016/07/multi-screen-...
This also was rather annoying and took a long time for distros to catch up: https://bugreports.qt.io/browse/QTBUG-42985
Since Plasma 5.8.x, it's much much better.
For different DPI on each monitor you'd need to use Wayland though.
mjg59 wrote a number of comments on this subject.
I wouldn't touch MacOS for it. It's nowhere comparable in productivity for me to something like current KDE Plasma 5 on Linux. Not sure if you ever used it, but Linux is nowhere limited to Ubuntu's Unity.