Ranked in these QuestionsQuestion Ranking
Pro Great Unity support
Since Mir is being developed by Canonical to fit the need of the Unity DE and Ubuntu, on many different devices, from desktops to laptops to mobile devices and tablets. Because of this, Mir is great for Unity, in many different ways (security, efficiency, functionality, etc.).
Much like the space station of the same name, Mir did not last. It's now been abandoned and will probably fall out of use, even with efforts by some to try and maintain it.
Con Created by Canonical to fit only their needs
Mir isn't bad. It just doesn't fit in the world outside Ubuntu products like Wayland does. Instead of Canonical choosing to use Wayland as their next generation display server they choose to go their own way, which does not contribute back to the community in any meaningful way, in that area.