Judging by repository history, Mir development seems to have been happening for several months in private and before Wayland protocol was deemed stable. If Canonical had different requirements, wouldn't it have been feasible to talk to Wayland developers and participate in it rather than come up with their own thing? Drivers are going to be a hassle as well.
Also, Unity is moving to become entirely Qt/QML based after a checkered history of changes. Unity 2D was only recently abandoned although it was written in Qt in favor of 3D which was Compiz based (which in turn required a lot of work to stabilize). Again, it seems Canonical would have benefited from talking to KDE folks a long time back since they claim to have the goal of a unified stack across devices which was aligned well with the KDE path.
In either case, these are very ambitious projects and I hope Canonical understands the development and maintenance burden of going solo on this.