To be fair, Wayland seems similarly hopeful about shipping in Fedora 23, which is only 4 months before Ubuntu 16.04. Also, Canonical has a big incentive to ship Mir in 16.04, since it's a long term support release. If they don't make it, they'll be forced to support X for an extra 5 years.

I think both projects have equally "hopeful" (slim) chances of hitting their dates. If they do ship on time, I expect stability and features to suffer.