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.
MySQL Group Replication isn‘t automatic in all cases, like starting after all nodes failed, and it has some limitations, but it is built-in.
Not saying that MongoDB ReplicaSet is bad, has been working very well for us AFAICT.