Hmm, my experience has been quite the opposite. I migrated from 3.x to 4 around... 4.2 or 4.3. It was way too early still. It took long for the desktop to get usable in even a very general sense. 5 though has been quite smooth. Some teething issues initially, for sure, but nowadays it's great*). Really the only two things I'm missing from 3.x anymore is the ability to configure the auto-hide delay for panels (I'd settle even for just a text file if it could be configured) and the ability to drag-and-drop a file from Ark to Konsole and have it extract to the directory the shell is in. I can live without the latter, and have learned to, but the first one has been missing since KDE 4. Still not gonna give up on auto-hiding panels though.
*) But since this is Arch we're talking about, I upgrade perhaps once a week, sometimes even less frequently. I might have missed a botched update or few. And since this is KDE we're talking about - I may very well use different parts of the DE than you do, I certainly do not use all of KDE.