Of course, but that "homework" would also include considering the incentives and goals of the authors behind the toolkit. If we took the authors of Qt as an example, their incentives and goals are clear. If they needlessly break their API, they will loose their customers and go bankrupt. These incentives have translated in one of the most smoothest major GUI toolkit transitions I've seen (Qt4 to Qt5).
As far as contribution processes go, both upstreams are equally tedious to work with. I doubt GTK would be willing to accept your code contribution if you added systray icons support back to GTK4 for example. So while the contribution process does matter, it doesn't matter as much as you make it out to be.