We've found the Postgres json store works just fine for our purposes, thank you very much.
Have you ever had an issue with conflicts, where multiple instances of the app read, modify and write different things to the same document at the same time?
This problem is what pessimistic (select... for update) or optimistic (using a version column) locking is for. If you don't want any race conditions to sneak into your code, as a rule you should probably be using one or the other regardless of whether or not you use postgresql JSON.
CouchDB and Couchbase both support only whole document updates. So you get document conflicts in those document stores as well, meaning your app needs to understand and handle 409's. But those conflicts are relatively easy to handle in most cases, at the cost of a new round trip. Mostly it's a matter of downloading the new document state and merging your change to it and re-post. If you're using Redux/Vuex/Event Sourcing this becomes trivial to support. Another way to handle it is to split a single large document into smaller pieces and write a map/reduce view that returns a composite document. That should be possible in Postgres as well with a prepared statement.