> I've never met an engineering team that used Riak

I was part of a recent cloud migration. Part of on-prem (though unfortunately not migrated by my team) were this very first Riak Cluster I saw in production.

The engineering team used it as "kind of S3" for images, with 3 to 5 PHP scripts providing an interface to Riak and imageMagic. It seemed to me like a good abstraction and I think the migration to S3 was mostly painless.

Other than that I only had contact with Riak at university around 15 years ago, when we tested cluster setups of several NoSQL databases and tried to manually introduce faults to see if they could heal. Riak passed our test at that time, MongoDB didn't.