However, I think there are challenges with this polygot data store architecture that he doesn't address. Each addition requires due diligence to understand its CAP trade-offs (which the author mentions briefly), scalability and performance characteristics, how to configure, etc. These are non-trivial concerns even for a single database. It's important to consider these challenges when building out a data store or data service.
I'd propose an architecture where the data services layer itself exposes different data models to the application, all of which are persisted in a single data store. Given that many data stores use a key-value store under the covers anyway, translating the specific data model down to a single, persistent data store would simplify operations while exposing the desired data model to the application. (As a caveat, this multi-model approach requires ACID transactions to ensure strict consistency when translating between data models.) This approach provides operational simplicity with just one data store and application efficiency by exposing the "right" data model API.
Full disclosure: I'm an engineer at FoundationDB, a database that provides ACID-compliant polygot data models on a single key-value data store (http://www.foundationdb.com).
