Comment 9 for bug 1499425

To clarify: All hope is not lost for nouveau. Nouveau works and it works fine with Mir in general. And mostly fine with Unity8 too. Nouveau is only unusable with Xmir in the default DRI mode.

You can avoid that problem however using -sw mode, and you can avoid the resizing issues using -rootless but only after the new xmir with working rootless gets released. Once that happens you've mostly only got general Unity8 bugs to deal with and can then use:
    Xmir :0 -sw -rootless -flatten

So please wait for the new Xmir to be released and let us know how that goes.