Like the parent said, Postgres supports unstructured data with json fields. The syntax is "clean enough" and you can easily migrate to a full column with a single query.

I like that I can easily build a view that includes "columns" from JSONB and normal table columns and clients are none the wiser. The clients get a consistent contract (the view schema) which can be backed by whatever tables make sense (or don't make sense but are used anyways).

I definitely appreciate the flexibility of a document store but I despise not having schemas for clients. Interface contracts reduce errors and ultimately make life easier for everyone. The database also has tons of information in RAM it can use to search and sort data, pushing those operations to the client balloons bandwidth requirements unnecessarily.

If you maintain clean layered code, accessing every collection via corresponding set of repository/data object classes - you will be just fine with MongoDB. But you better do that with SQL database too, anyway.

Mongo has schema enforcement, and if adding index it will keep and serve data straight from RAM.

Having used Django a lot in the past, migrations aren't that much of a problem with a decent tool. Mongo is still quite crap to work with in general.