I led a migration from Mongo to Riak at Shareaholic about 12 years ago: https://www.slideshare.net/slideshow/migrating-to-riak-at-sh...
It was successful at first, but ultimately we traded one set of problems for another (how novel, I know).
In particular, I underestimated the pain of troubleshooting the database itself. Riak was a new product, we were a small team that had never run anything on BEAM, and ultimately we lost too many days debugging and trying to make sense of Erlang stacktraces.
The Basho folks were great, and to this day I appreciate how quickly they fixed a number of bugs for us. But ultimately it wasn't enough -- we found problems faster than they could be patched.