Is KDE 5 Stable? It's Complex
Martin's post basically boils down to that the Linux desktop stack is complex, lower-level issues often get blamed on the desktop, and distributions end up being different and forming the software stack of their choosing.
Some of the causes for problems associated with the modern KDE Plasma 5 desktop are due to poor Linux graphics drivers, the lack of QA/testing on as many graphics driver combinations as possible, multi-screen issues with X11 caused by Qt, etc.
There's also the matter that for some KDE Plasma 5 issues they could have been fixed months ago but have yet to reach users on non-rolling distributions. Also, some users take issue with some features that were removed or streamlined as part of KDE Plasma 5 / KDE Frameworks 5.
If you want to read more about Martin's view on the whole "KDE 5 quality" debate, you can do so here.