I have installed the latest version of watchman, which can be found here:
/usr/local/bin/watchman
I performed the Install Recommended Packages on Startup and I watched them install after restarting.
This is a fresh new install of Atom and Nuclide with the latest version of everything.
I have a test project with files added to the Mercurial repository. The repository .hg directory sits at the root of the project. When I open a Diff View into a file, Nuclide picks up the previous Mercurial version of that file just fine as I make edits and it shows the comparison between the two. Previous version on the left is shown. So I know that Atom-Nuclide is able to interact with the Mercurial repository.
However, nothing else seems to be working for Mercurial support.
When I select "Toggle Blame" on the same file where Diff View is working, I get this message: "Failed to fetch blame to display. The file is empty or untracked or the repository cannot be reached."
The File Tree Highlighting does not work. No colors on any of the files in any of their mercurial states.
The colored Line Modification Notifications do not show inside of the Atom gutter.
The Added and Removed Lines feature is not showing in the status bar.
These features in Atom-Nuclide are the reason why I am interested in trying out Mercurial instead of Git and are big reasons for trying this IDE in general.
The same problem was reproduced on a different distro. I can't be the only one who bumps into this. No business can safely rely on a development environment where the level of community interest outside of FB cannot support an attempt to claim a Stack Overflow bounty on a question like this. The solution is to wait for better type support and type checking to come to PHP. Numerous RFCs exist to do exactly that and other IDEs will take advantage of this in future versions of PHP.