I also contend the supposed "ugliness" of gtk. This is at most a personal issue; in my view gtk programs are typically beautiful and Qt and Electron are unbearably ugly.
Definitely depends on the platform. On Linux it's probably great, but on macOS at least, I've yet to see a GTK app that doesn't feel super clunky. GTK breaks lots of expectations about how things should behave on macOS (shortcuts, standard menus, buttons, etc). Gimp or Inkscape are good examples. On the other hand those are examples of what a "native" app is expected to look like:
Note that the apps above are not implemented with Qt or Electron either, they use the official Apple frameworks.
GIMP and Inkscape are still using the ancient GTK2, GTK3 is generally a lot nicer.
i.e. Transmission uses GTK3 and looks like this[1] on OSX.
[1] https://en.wikipedia.org/wiki/Transmission_(BitTorrent_clien...
#/media/File:Transmission_v2.92_on_macOS.png){kind=link}
> A native Mac OS X GUI application
> GTK+ and Qt GUI applications for Linux, BSD, etc.