I have used Fedora + KDE for the past 5 years. Yes there are occasional issues but they are normally common to all distros which happen to use the same combination of components.

For me, I ran into a regression with compositing: https://bugs.kde.org/show_bug.cgi?id=354146

After I disabled the desktop effects Fedora 25 was rock solid.

I haven't upgraded to F26, I normally wait a week or two to shake out any big issues.