The article focuses on these two database technologies, but in fact it can be extended to the whole realms of RDBMS and NoSQL databases. Certain aspects of both domains can be considered as advantages or disadvantages, depending on the point of view and particular business case. The flexibility of schema in MongoDB is appealing to teams who implement applications dealing with unstructured data. The strictness of schema definition required by relational databases can be beneficial when the application needs to ensure that the data quality meets certain requirements.
Relational databases have been with us for a long time. They are not going anywhere, don't worry. There is however a category of problems, for which the relational databases are not best suited. The technology development in recent years allow the graph databases to emerge from academic research and theoretical concepts.
The in-memory database technology can be a game changer, especially when the technology will become a commodity. At present the IO is the biggest bottleneck of the databases, because most of relational database systems require transactional consistency of writes.