I enjoyed using Phabricator but the barrier to entry is too high for a community project like KDE which needs every new developer it can get. Just a few small hurdles are enough to put people off.
Another thing that we hear from people considering a move to GitLab is that they really like how fast GitLab moves (one release per month!), and how it works with the community to keep improving the product. I recently joined as the Sr. Open Source Program Manager as part of the Community Relations team, and we're thinking a lot about how to continue to improve the experience for our community.