I used Riak for a project back in 2012, the app that became the Whisper App, and as a huge Erlang fanboy, I was so excited about it.
But it was incredibly unreliable at scale, and my colleague and I spent a week of sleepless nights under incredible personal and business pressure - as the servers got busier and busier - ripping it out.
Still love vector clocks, though, and have fond memories of the Basho team presenting at Erlang Factory
Vector clocks are very cool. Having read through how they were initially used in Riak, I was blown away that such an implementation could scale. I guess this is why Cassandra took a different approach?