I have used quite a bit of Gtk and QT, and have had to touch X11 or Wayland very little directly, EXCEPT for one case where I wanted to provide a global hotkey...
Which is kind of understandable as Wayland tries to be more secure: and thus in Wayland not all keyboard events are propagated to all applications (that's what X11 does). I think it's a good idea to put security first in this iteration of FLOSS desktop technology.
Well kind of. It'll be several decades before we see any practical benefits - at the moment once you have local execution you can do anything you want - accessing other apps or even root is trivial.
Phoenix[0] has some good ideas about how X11 could be made more secure without breaking backwards compatibility. I don't understand what was so fundamentally broken about X11 as a protocol that it required a replacement protocol.
We can argue about limitations of X.org's implementation of the X server, but, as demonstrated by Phoenix, X.org doesn't have to be the only X server implementation.
...which is overkill when you only need a Vulkan or GL canvas which spans the windows client area... and even with GTK or Qt your app still stands out like a sore thumb on the "other" desktop environment because the window chrome doesn't match the rest of the system.