The problem I see is that for RDBM’s to scale you have to be a witchdoctor. You have to ruthlessly normalize and tune. Usually, the tuning that you have to do to achieve performance comes at the cost of maintainable, reusable application code. Often I have had to alter a clean, reusable object model in order to get performance out of the database.
When having to scale a relational database you are slowly removing the relational parts — I’m not referring to removing relationships only, but also denormalizing, etc. which leads to emulating a basic document store or key-value store on top of a storage engine meant to something else.
Original title and link: Relational Databases and NoSQL Databases (NoSQL databases © myNoSQL)