I have used JSON types in Postrgres for a little while now, they're pretty great! I can't really see a downside, is there a disadvantage to this kind of setup?

In our use case there were very clearly parts of the data that were relational and other parts that were better represented as unstructured. Postgres gave us the chance to do both in one place and worked really well for that purpose, good to see it getting even better.

Read somewhere that postgres cannot do in-place json updates. Like you cannot do field->>'val' = 1 in it without updating the whole json.

Which becomes relevant event in moderately write heavy scenarios

I think this is possible as of 9.5. See `jsonb_set` here: http://www.postgresql.org/docs/9.5/static/functions-json.htm...

Not sure how it is implemented, though.

9.5 adds a bunch of functions for modifying (setting, deleting) nested data.