I evaluated phabricator today for Hg hosting at $DAYJOB. Issues in Maniphest aren't "attached" to a Diffusion repo, so you can't use it as a Gitea/Github-style one-repo-one-project system; it's one-phabricator-one-project-multi-repos. Spaces help a little bit but not really.
Dealbreaker for us - we'd rather retrain staff on hg->git (and lose the amazing TortoiseHg).