NoSQL is much bigger than Mongo, which deservers books written on hating it. The relational algebra is great, but its also overkill for things like event streams, message brokers, caches, does hit scalability issues for certain legitimately large workloads, and in comparison to Mongo and other document-based databases, fails when your data simply is not relational.
Then there's the whole object/relational impedance missmatch, which if you insist on using OOP, there's a variety of object databases that make a small persistance layer much easier.
One case is where you have fairly ragged entities that may have several of many attributes but result in very sparse rows.
Another case is when users are effectively designing their own schemas but you want to keep them, for example I have a database for a CMS where users can define nested hierarchies of keys with translations that would be impossible to fit into a schema. You could just toss this data into a JSON column, but its an example of nonrelational data.
