I mean that this persons code is not production-ready. As in there is no chance it emulates all of the features of mongo or has been performance tested etc.
http://www.postgresql.org/docs/9.4/static/datatype-json.html
1. Can't sort using a GIN index.
2. Stats are shit with GIN indices - Postgres assumes that every filter has 100 matches, every array has 99 elements or something. Usually this works well, sometimes (specifically in my case) it's terrible. Specifically, it means that if you combine an aggregation over a GIN filter with an inner join, the performance will be awful because postgres picked a nested loop join for expected 100 results instead of a hash join for the actual 50,000.
3. This also means that additional expression indices you add basically aren't getting used, because it'll always appear better to the query planner to use the GIN index first. This includes things like sorting.
4. Because of how jsonb is implemented (basically arrays containing fields ordered by key alphabetical), we discovered that we were taking out more and more fields out of our jsonb field and placing them in other columns, which kinda missed the point of it (but was very good that we could do so, yay relational data model).
5. Sorting can become an epic chore if you don't have stuff like enum types etc, especially when you try to combine it with tsvectors.
So, very happy with Postgres, but don't slam people who don't use Postgres because of insufficient json support - because Postgres' json support although excellent has quirks that will bite you with certain use cases. Very good, but not a panacea.
2. That's (probably) occurring because the default statistics target is 100. Just increase them with:
ALTER TABLE gin_index ALTER COLUMN indexed_column SET STATISTICS 1000;
That should hopefully help with all the other issues you are seeing.
P.S. while looking at MongoDB a little further, I discovered that it doesn't support sorting by collation. This was logged as a JIRA bug in 2014, and as of yet I don't believe there is a solution.