I'm glad that @lah7 who is a dev and seems to be a level-headed person brings this up. Maybe the end of support for UM 16.04 is getting closer and closer? (or maybe it was just three years so it's already EOL).
I think it's fair to say I left MATE for Xfce because of GTK3. Now that Xfce has been ported to GTK3 I took a look at it in Xubuntu 19.04. I felt the same as when MATE became GTK3. Why would I want to use this?
Xfce will still be GTK2 in the upcoming Debian Buster. That's my mental escape route. Because I can't see why I would want to use Xfce GTK3 instead of Xfce GTK2.
One crazy aspect was the built-in color-chooser which had the useless stock GTK3 design. This has been fixed in MATE I believe so I don't understand why they didn't use that. Lack of time probably.
If it wasn't for headerbars and Client Side Decorations I think "GTK3" could be implemented as a CSS theme engine for GTK2. Anyway, GTK2 will live on in KDE and LXQt thanks to the gtk2 Qt widget style which is good at mimicking GTK2 themes.
On my GTK2 system I easily spot GTK3 apps, but it can be difficult to spot Qt5 apps just looking at the window, unless you know what it is. I never use apps with headerbars or CSDs. It doesn't feel right and I don't have to because it's mostly Gnome apps anyway.
The GTK3 applications I use: SeaMonkey (dying?), Firefox, Thunderbird, Synaptic, MATE Calculator, Atril and Eye of MATE. These are good traditional applications.
Only GTK3 downside I notice in those applications is that scrollbar in Atril and Eye of MATE sometimes doesn't move and sometimes isn't proportional to document size.
I think GTK3 can work just as well as GTK2, but there are often small UI/theme problems which we didn't see with GTK2. Clearlooks - the default theme for Gnome 2 - didn't look good until the end of the Gnome 2 era. So these things take time and constant GTK3 changes make it harder.