Riak is horribly unfriendly as a database: no SQL, it exposes eventual consistency directly to the developer, it’s relatively slow, and Erlang is a fairly unusual language.
While you can run Riak on a single server, you’d have to really want to.
Its strength is the ability to scale massively, but not many projects need that scale, and by the time you do, you’re probably already using some friendlier database and you’d rather make that one work.
In the end more and more data was offloaded to MariaDB, until one day the last remaining data couldn't justify the cost of the Riak cluster. I think we swapped out an eight node Riak cluster for two largish MariaDB database (one being a hot-standby).
For one of the other clients it was the exact same scenario, only we had been contracted in to help run the Riak cluster, which we didn't do well. Once they had migrate of it, to Oracle I think, the client left.
To me it always felt like it was just the wrong tool for that particular job. Someone really wanted to be able to jump on the NoSQL hype and sell something. They picked Riak, because it honestly looked really good, and probably was, compared to MongoDB, CouchDB or whatever else happened to float around at the time. It just wasn't the right tool for the problems it was applied to.