I'm a postgres fan but I have to admit the JSON syntax is not the best and not well supported by all the tooling around this database. Doesn't mean it's bad but the DX could be better.
Other than that, I have never seen a proper justification to use schema less db, good for prototyping and get started, not incredible as a long-term solution.
Schema-less is a bit wrong term, since you always end up with one but, indeed, schema-on-read vs schema-on-write discussion for individual use cases is far from a settled thing, even though the zeitgeist is that schema-on-write won sometime in the 70s.
You say "prototyping", but I would generalize that to "faster evolving" in the long term. Of course, not without tradeoffs.
I personally have zero issues writing a custom marshaller when needed for any the schema-less document collections I have. Constraints your application has on the data are a superset of db schema anyways.
Other than that, I have never seen a proper justification to use schema less db, good for prototyping and get started, not incredible as a long-term solution.