This is a post from slashdot, which I think may have a point.
By contrast, the Qt based LXQT 9.0 seems familiar, minimalist, and well organized.
GTK+ is rife with serious problems. The first is that it's affiliated with the GNOME crew. Their grasp of sensible, proper UI design is very suspect, especially after the GNOME 3 disaster. For example, these are the kind of people who took gedit, GNOME's text editor, and changed it from this sensible, usable UI [wikimedia.org] to this hideous, unusable UI [wikimedia.org]. You can even see a screenshot of this shitty UI in the Xfce 4.12 tour! It has, sadly, been infected by this bad UI design.
The portability of GTK+ is, to put it politely, utter rubbish. X11 is the only platform where it isn't a disgrace. It "works" under Windows and OS X, but if by "working" you mean it runs but is generally unusable. I haven't been able to ever get it working properly under OS X. It didn't even get to the point where it showed a UI, the last time I tried it. Inkscape is horrible. GIMP is horrible. Every other GTK+ app I've tried on Windows or OS X has been absolutely horrible.
By contrast, the Qt based LXQT 9.0 seems familiar, minimalist, and well organized.