json documents sound great, especially initially, but end up being a maintenance nightmare
The problem with that - in my experience - is that migrating the structure for thousands (if not millions) of documents is way slower than running a DDL command (as it means reading each document, parsing it, modifying it and writing it back). Many DDL commands are just metadata update to the system catalogs so they are quite fast (e.g. adding a new column with a default value). With documents you wind up with millions of single row updates.
This can be mitigated by doing a "lazy" migration when a document with the old structure is first read. But that makes the code much more complicated.