You can have well defined data and use Mongo. Those two things are not related. I worked at a place that used Postgres and put an object w/ 11k unique paths in a single JSONB column with no schema or documentation whatsoever.

... and this system was responsible for billing, and many other things....

and? That's not PGSQL fault, is the DB architect (or lack of) fault. You definitely can drive the safest car in a ditch, no?

That was my point right? It has nothing to do with the DB.

Also, MongoDB queries for related records can be painful. This takes a while to realize, but definitely shows itself when you are working with more complex data.