Imo Linux, KDE and most other software works nicely with HiDPI. A small issue is GTK apps that don't respect DPI settings. The most significant problem AFAIK is for people who switch frequently between different displays. There are a few settings here and there you have to adjust every time you switch between Hi and Lo-DPI.
Having written some Qt apps and used them on scaled screens... eh. It's got some work to go, but we're making progress. Apparently Qt 5.4 will have further improvements in this field too.