I'd love Ubuntu to part of that love in, alas they've clearly made a bid to control the entire FOSS graphics stack longer-term.
On the surface it doesn't look that bad, look much closer at the way the project (& licensing's) structured & it becomes clearer.
And the reasons they cited at the time for such a schism being needed have long since been thoroughly lampooned.
They have some of their own reasons (which they've since clarified further) & that's fine, they're entitled to do what they want.
But they need to get off their high horse & stop pretending Mir's mostly for altruistic reasons & the greater good, that's not what it's about at all.
Lately they seem to have been doing just that, but only when they're not on a "Jobsian" marketing drive of course...

There's been excellent & detailed discussions on all this for many mths now in threads tied to articles @Phoronix, & elsewhere.
Might be time to tune into those if you're interested in more than just Sailfish, Dave?
Tizen's a bit better overall but not by much*, & putting aside the openness Qn, it's disappointing that native is not truly central like HTML5.
It's there, but it's not "front & centre" to their overall approach, as evidenced by the fact that native dev is well supported via C++ only.

*most of the OS components get a big tick, but there's some (surprisingly) very tight restrictions when it comes to the officially mandated/supported SDK

Yeah but, that's a prototype, Jolla's already confirmed (yonks ago) that they've started pre-production, that's a BIG difference.
Admittedly, most of the images (some are renderings no doubt) indicate a ph that's pretty slick when it comes design, perhaps more so than Jolla's.
Some of that flair for marketing coming to the forefront for Canonical again, longer-term it certainly helps more than it hurts.