Having used the 4.3 version I wouldn't recommend it, specially since looking at the web site it seems anything has hardly changed in that area since my attempt.

It adds very little value over Java + NDK on Android.

The Android native widgets aren't supported, rather emulated via QML and only the most well known ones.

For the majority of Android APIs no support is provided, offering no advantage over we having to write our own JNI wrappers.

Also iOS and Windows Phone support are even worse than Android.

That's a strange complaint. The whole premise of putting QML, or any other UI toolkit for that matter, on Android is to use those instead of native Android widgets. In that sense, it adds a lot of value.

The premise of a portable framework is to allow me to write my code in a portable way across multiple operating systems.

If I have to spend time writing bindings to OS APIs and faking UI widgets for each OS I am targeting, then the framework is not doing anything to help me, rather on the contrary.

As extra info, on 5.3 some of the C++ widgets render on Android as if it was a desktop. Just try to use a Qr file selector on your phone.

Apparently this has worked so well that 5.6 is going to have yet another re-write of the controls.

The purpose of porting alternative UI toolkits is to let developers use their preferred programming language. Users don't want to see UI widgets that look out of place. Ideally there should be no perceivable difference in the end result.