"The technical core of the article is the comparison of MongoDB and PostgreSQL, along with our experience in switching between them. We were motivated to change systems primarily for performance reasons: many searches that we wanted to support stalled on common inputs. We did not see a way to resolve the issue within MongoDB. Several missing features in MongoDB also pushed us toward shifting: support for arbitrary precision integers and transactions that allow multiple commands to be rolled back if there is a failure."
That is a pretty old paper if they made the switch before MongoDB 4.0 was introduced. 4.0 was released in 2018. One from the archives.