Mir 2.2 Released With Display Wall Capabilities, Composite Bypass On GBM-KMS

[Mez']

The fact that Ubuntu uses Mutter by default and not Mir says enough. Canonical knows how useless Mir is.

Not exactly. Mutter and Gnome are too imbricated. If I'm not mistaken, you can't use anything else with Gnome, it wouldn't work if you remove Mutter.
If it was possible, I would have gotten rid of Mutter.

I think Budgie intends to develop its own Mutter replacement though. Since it will be on top of GTK as well, it might be something to look into.