We occasionally managed to display a decrease in open reports in the bugzilla weekly reports; I remember we did during at least one annual hackathon.
Besides, in Phabricator we have no way to tell bugs from feature requests from random discussions https://phabricator.wikimedia.org/T102#11218 , nor reports/charts like https://bugzilla.mozilla.org/reports.cgi?product=Bugzilla&datasets=UNCON... nor a real advanced search for all task metadata and history.
We could maybe produce dumps of Phabricator data and import them in a bugzilla instance regularly to ease research, but see https://phabricator.wikimedia.org/T108199
Nemo