One example would be KDE, who considered Gitlab but ended up going with Phabricator because they are one of those 100+ developer projects that wanted the LDAP, analytics, logging, etc but also being a major open source software community wouldn't settle to not dogfood free software, and the CE was missing too much at the time.
Personally, I much prefer Gitlabs project -> code / issues / milestones / etc workflow over Phabricator's git / issues milestones -> projects workflow, and love using Gitlab, but there are a lot of free software communities (Gnome, Freedesktop.org, among others) who are almost certainly going to try moving to more modern project management services in the near future and will be forced away from the IMO best UX Gitlab offers for ones like gogs / phabricator that both sate their needs for a free software solution that also has the featureset for these thousand+ developer ecosystems.