All right, KDE Neon on 18.04 runs.

After the upgrade from the previous KDE Neon version, everything seems to run smooth.
There was one problem with a library referring to Chines input. I removed that library and all is good. Reinstall input fixes it.
Same was with Emacs 26.1. I had to recompile ist and install it via the make command. Now the missing library is installed, all is good and running fine.

Well that was the least to be expected with that long time waiting for the update.
If I find a secure method to switch this install back to Ubuntu, I'm out of KDE Neon. I want to have the latest and greatest of Ubuntu. It's even more important to run an old system for month with the latest KDE.

For now, I'm fine.