The first version of Joplin was done with Qt and QML (the code is still there in the repo in old commits, before it was replaced by React), but as mentioned it’s not great for mobile. Also when I’ve started QML was recommended over QtWidget, yet the library of components at that time was very limited.
For desktop I would have preferred using Qt actually as I’ve developed before a relatively large cross platform app with QtWidget and it’s a very good framework with excellent doc.