There's really nothing simple about building apps on top of Riak. It's one of those things that seems simple until you use it in production, and then you realize it's a total nightmare and you can't wait to sunset it.
Without details, I am not really sure what you are talking about. Riak is just a key-value store, probably the simplest abstraction to store data. I have put it in production several times and operated it for years. Reliability and performance is just outstanding. Scalability was capping out around 150 nodes in the same cluster.
