Postgres is not an alternative to Mongo, though. Provide a noSQL alternative if you want an alternative. RDBs and non-RDBs seek to address different sets of problems, and sometimes you do not want or need a relational database.
But postgres actually is that with json and jsonb. The postgres developers are also very focused on providing a better alternative to mongo.
Afaik similarly configured postgres also challenge or beat mongodb in performance.
Setting up a simple cluster without relying on addons, or having to worry about which server in the cluster your writes are going to (bisected application configurations)
