Canonical is an interesting case. While all the others (except maybe Mozilla) do a lot of work on the systems programing, most of Canonical's system level stuff is done by many smaller, upstream, projects and Canonical is responsible for integrating them, and selecting ready for release versions. This allows them to much easier avoid to early to release problems, and direct their effort in a much more focused way on the end user experience.

Also, their duel release model (long term support and 6 month short term support) gives them a nice test bed where they can try out radical re-designs before forcing it onto the users, who can continue using the LTS release.

I don't think Apple or Google is doing anything nearly as complex as Windows 8, once you factor in the number of hardware partners and the constraints on backwards compatibility.