Mir is not a good example of distro engineering, because it’s an extreme case of NIH syndrome. Unlike what it is today, the original Mir was an alternative to Wayland.
The story started when Canonical decided that X isn’t good enough and they needed an alternative. They chose Wayland first, exciting the entire Linux desktop community. But then they dropped Wayland in favor of the new in-house Mir project, citing several drawbacks to Wayland. The Wayland community responded with several articles explaining why Canonicals concerns were unwarranted. But in typical Canonical style, they simply neglected all the replies and stuck with Mir.
This irked the entire Linux community who promised to promote Wayland and not support Mir at all. This continued for a while until Canonical realized their mistake late, like always. Then they repurposed Mir as a Wayland compositor.
Now this is a repeating story. You see this with Flatpak vs Snap, Incus vs LXD, etc. The amount of high handedness we see from Canonical is incredible.