I used Phabricator on a daily basis at Facebook, and loved it, after having used it at a previous job where I hated it. The biggest reason for that was that Phabricator is a piece of software that wants to be used in a very particular way, and that is the "you're all in" sort of way. It's not a tool you'll get good results from by timidly testing the water with your toe.
One particular manifestation of the prior point is that, for me, it worked best with a one-commit-per-unit-of-work workflow, where stacks can be merged on a commit-by-commit basis and code review issues are addressed by editing the changes into the relevant commits. In Git, making this work is a painful amount of history rewriting that lands you in a world of rebase pain. In Mercurial, you just fix things at the top of the stack and `hg absorb` the changes into their relevant commits. `hg absorb` is, in fact, the one Mercurial command I really wish I had in Git.
One particular manifestation of the prior point is that, for me, it worked best with a one-commit-per-unit-of-work workflow, where stacks can be merged on a commit-by-commit basis and code review issues are addressed by editing the changes into the relevant commits. In Git, making this work is a painful amount of history rewriting that lands you in a world of rebase pain. In Mercurial, you just fix things at the top of the stack and `hg absorb` the changes into their relevant commits. `hg absorb` is, in fact, the one Mercurial command I really wish I had in Git.