Note that the wheeldesigners are just making a new default theme. With minor tweaks all kde 4 series themes are still usable.
Also, the inconsistencies are mostly due to app developers themselves. The difference between plasma and the software compilation can exemplify that - you use two entirely different application stacks if you are developing for one or the other (qt widgets enhanced by kdelibs vs qml enhanced by plasma). And then there are multiple ways to actually develop the app interfaces - you can use anchor based widgets, ui files, or raw custom positioning which I find in reviewing a lot of codebases a bit too common for my liking.
When I have the time I try to fix it, though. The problem right now is that while the Qt world's future is QML based, the KDE applications and libraries were developed for over a decade around widgets. So when I develop something new I'm using QML and it can scale, animate, and orient really well for arbitrary window sizes and devices, but some widgets applications just anchor a toolbar to 50px indents and then it looks like puke on high DPI.
I mean you can still do that in QML, but the anchors and layouts are so easy to use it seems insane to do so.