We used Mongo for the first iterations of our app, but the relational nature of our data was an awkward fit for a database that is not relational. We sorely lacked relational database integrity features that needed to be done on the application side (poorly) and it was a huge relief when we managed to port our application over to Postgres, which performs great and never gives us trouble, while having very user friendly extensions like JSON and PubSub that made the transition easy.