Keith Packard's "Repository Formats Matter" post nicely captures how meaningless it is to focus on this sort of thing in the long term: https://keithp.com/blogs/Repository_Formats_Matter/
I.e. yes Git has some UI issues, but those are fixable, whereas e.g. Subversion's UI was way better than Git in the early days, but its repository format limitations inherently weren't fixable.
> Mercurial has a concept of commit stages to make history rewriting safer.
Yeah that's a really neat feature. For what it's worth some people at Google seem to be working on trying to get an equivalent feature into Git.
> For example, what Facebook did[...]
Much of what drove Facebook to Mercurial has since been addressed, e.g. "status" times being slow due to lack of inotify-like integration. That's now a feature of core git. Some of the rest is being worked on and actively upstreamed, e.g. from the GVFS effort: https://vfsforgit.org
