Keep in mind that JSONB is different than JSON in PG. The later will not transform the JSON simply check consistency and has limits and performance hits.

JSONB is encoded and will transform your JSON, it will drop duplicate keys and order might not be preserved. But it's also faster and easier to use (IMO).

There is no real reason to use MongoDB tbh, PG JSONB can be handled like any other field and you can even index into JSON (partial indices even; only index rows where a field is present in the JSONB)