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.
One of our biggest disappointments: we had plans to add a way to enforce strong consistency leveraging (IIRC) something akin to multi-paxos, but couldn't get it to work.
The engineering exodus around that time sorta killed the project though, and we never were able to do the big follow-up work to make it really shine.
(Disclaimer: Former Basho Principal Engineer, primary author of strong consistency work, lead riak_core dev from 2011-2015)
I think another 18 months would have been enough too. But it just wasn't the right environment after the hostile take-over / leadership transition.
I apologise if we do eventually cut it. Having worked through the code when chasing unstable tests, I developed an appreciation for the quality of the work.