Well, it was buggy until not very long ago:
* 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.
Meanwhile, multi-monitor support in Plasma 5 has worked for me for years without any trouble whatsoever.