KDE Plasma 5 is actually at a great state right now. It works well and you can customize it to me whatever kind of desktop you like. Canonical could have made it into whatever they wanted it to be.
It's built on top of a Qt, that works everywhere instead of GTK that just focuses on being there for GNOME. In addition to that canonical has already been using Qt for the past few years and could have just continued on with it. Alas, the decision has been made and they're not going to be changing their mind anytime soon.
I don't think KDE is really quite there yet. For me and from what I've seen and heard it can be extremely unstable (and that's using KDE Neon which ships the up-to-date version of KDE). Hell, at this point I'm giving up on S3 sleep and KDE because after starting it up again KDE is sure to crash a lot when doing simple things like switching to other apps etc.
I'm using Plasma 5 and I cannot confirm any of this. Things have been very stable ever since I switched to it (from Plasma 4) about three years ago. Much stabler than Plasma 4 in its early days (that was, admittedly, a huge crash-fest). And S3 sleep is causing literally zero problems.
It's built on top of a Qt, that works everywhere instead of GTK that just focuses on being there for GNOME. In addition to that canonical has already been using Qt for the past few years and could have just continued on with it. Alas, the decision has been made and they're not going to be changing their mind anytime soon.