I know there are a lot of bad vibes on HN about MongoDB, but I also know several companies which MongoDB enabled with the right libraries to much faster iterate on "schema" and business requirements (in their growth phase) than with other databases at that time (some years ago). The schemaless approach of MongoDB suited their fast changing needs much better than e.g. RDBMs.

What currently still makes MongoDB nicer than e.g. Postgres JSON are the libraries embracing schemaless while JSON in PG libs still feels tacked on.

Recently I had some bad experience with MongoDB support though.

No experience with RethinkDB.