Is there a benefit of Mongo for that over just Postgres with JSONB columns? You're still storing JSON in Postgres but you'd get the relational aspects too for things like users having many forms, billing and account relationships etc.

Easier horizontal scaling and organizational inertia would be the main reasons to use mongo over a jsonb column. I wouldn’t introduce it to a psql shop if they are already great at running psql.

No benefits that I can think of at the database layer. Postgres's addition of JSONB columns represents the best of both worlds. Funny to think that all of that noise about nosql replacing rdbms was essentially nullified by the addition of one column type.

Some people do like the MongoDB API, however.