I used Phab at a job and it was a complete mess. Stacked Diffs only sort-of worked, CI integration was bad, notifications were so noisy everyone tuned them out, etc.

Talking with a Meta person, it sounds like Phab really needs Mercurial to work well, at least for Stacked Diffs because you need to be able to identify commits independent of their location in history to properly maintain the Stacked Diff associations.