I believe fragmentation is a very big issue in the Free Software world. Since both Mir and Wayland wanted to work towards the same goal with minor differences in details, they should have banded together and have had proper discussions regarding both their ideas. It encourages better solutions and sometimes new things are born.
Mir was also never as performant as Wayland and didn't have any window manager apart from Unity to test with.
Unity:
I personally don't have a thing against Unity but there were (16.04 days) some inconveniences like being unable to customize the dock. The separate toolbar and menu bar. GNOME does have thicker titlebars but combines toolbars, menubars and titlebars in their native apps.
GNOME also has a rich ecosystem of extensions. (Please stop breaking them through updates GNOME!!!). GNOME has a better universal search than Unity, is comparatively faster and snappier than Unity.
GNOME suffers from the problem of poor defaults which are easily fixed by distribution vendors. (Kali Linux ships with a dock, topbar and other IMO necessary extensions.)