I don't know the specifics, but most of the backlash against Mir is political (and rightly deserved). Canonical had originally stated they were going to support wayland, like most other distros, but were suddenly overcome with a huge case of NIH and decided to make their own. While in the early stages of development Canonical released some "technical information" which basically blatantly lied about wayland and it's capabilities.
Mostly Mir exists because Canonical had a huge case of NIH.
For completeness: Canonical still claims that they couldn't do what they wanted, especially with mobile devices, using Wayland. Or at least, they would have needed to write various non-standard extensions to the Wayland protocol which would make it incompatible with vanilla Wayland. The best source for this is this blog by someone working at Canonical: http://blog.cooperteam.net/search/label/mir
I don't understand graphics systems enough to evaluate their claims, and I've yet to hear an impartial analysis of them. Possibly no-one who can evaluate them is impartial.
Mostly Mir exists because Canonical had a huge case of NIH.
Sorry for not having any technical information.