Nico,
The two I’ve found references to are the platofrm-specific limitations (which aren’t all that limiting)
https://doc.qt.io/qt-5/qsettings.html#platform-limitations
As well as what groups can contain when it comes to special characters, I think there are differences in groups containing slashes; and that’s where I’ve seen trouble.
Beyond that, it might just be possible, for most cases, to load one file with QSettings and output it with another format by explicitly setting the file format.
-Kessler