I’d say that another major feature is that the maintainer of the toolkit is not interested in making sure the open-source version receives as narrow a use as possible. (The current maintainers of Gtk also did a fairly good job ensuring that, but at least it wasn’t an overt goal...) That ugh feeling from Qt, along with being forced to deal with C++ if I want to dive into the system, is the primary reason I haven’t switched to KDE, despite liking both their UX philosophy and their view of their place in the ecosystem[1] much more.

[1] https://youtu.be/IFGXVN9dZ8U (I don’t think there’s a text version, sorry)