Phabricator has been in use as a production tool for years and it shows. It is much more sophisticated in terms of workflow and real-world use than alternatives.
Note that not all of those use Phabricator as a bug tracker (KDE, lighttpd, LLVM, Haskell, I imagine a lot of bugs for Wikimedia are reported through project chat pages on the wikis themselves).
An incomplete list of projects using it, some of them VERY large:
- Blender (https://developer.blender.org/)
- LLVM (http://reviews.llvm.org/)
- Haskell (https://phabricator.haskell.org)
- Wikimedia (after lots of discussion and a lengthy selection process, https://phabricator.wikimedia.org/)
- FreeBSD (https://reviews.freebsd.org/)
- Fedora (in some places, for example: https://phab.qadevel.cloud.fedoraproject.org/)
- Khan Academy (https://phabricator.khanacademy.org/)
- Enlightenment (https://phab.enlightenment.org/)
- KDE (https://phabricator.kde.org/)
- Freedesktop (https://phabricator.freedesktop.org)
- lighttpd (https://review.lighttpd.net)
- Kolab (https://git.kolab.org/)
- ...
Wikimedia hat lots of content about their choice and the migration process: https://www.mediawiki.org/wiki/Phabricator/FAQ
Phabricator has been in use as a production tool for years and it shows. It is much more sophisticated in terms of workflow and real-world use than alternatives.