They work for a very specific use case. And if you need to expand that use case, or address others, good luck. Relational database systems are complicated for good reasons. Schemas, which do cause problems, are there for good reasons, and the alternative problems obtained by going without schemas are just not worth it, nearly all the time.

I think a lot of the attraction of NoSQL databases is that they are seductive for people who don't know about the problems inherent in working with shared, persistent, long-lived data, (i.e. years). A NoSQL system lets such people get started easily. They start down the path, and then they run into dragons, and muggers, and hostile aliens. This is a really good description of what I mean: http://www.sarahmei.com/blog/2013/11/11/why-you-should-never....