Testing shows that it works better than Qt 5.8.0, but that's rather easy.

Some regressions introduced by 5.8 (some wayland related) aren't fixed yet and Qt WebEngine does not build on AArch64.

There is a bug that breaks applications using kdialog horribly and in the 5.9 branch (to-be 5.9.1) there's already a change that changes behaviour in a (incompatible with the 5.9 doc!) way that breaks some existing applications.

At least they can't make any excuses to not release 5.9.x releases this time...