If you rely on Postgres and/or Mysql replication then you'll need a DBA on call. Lots of manual work involved in babysitting them when stuff goes south. Mongod

If you rely on Postgres and/or Mysql replication then you'll need a DBA on call. Lots of manual work involved in babysitting them when stuff goes south. Mongodb is entirely automatic, you just delete bad servers and reprovision them automatically and everything "just works" with no interruption.