For example, I worked on a IoT project for an R&D company and Riak was definitely a better choice than a SQL database for our usecase : Easily distributed, handle large very large amount of small write and small amount of large read. It was also a necessity because we could generate several gigabyte per minutes and being able to add Riak node to add more storage space and bandwidth dynamically was very useful.

But yeah, SQL DBs are like a good toolbox : They can be used in most situation and will work fine. But "NoSQL" DBs are usually more of a dedicated tool for a specific use : It's better for this specific case but might not do well for other tasks.