Riak is not great. It performs well, but doesn't scale well. It has tons of operational overhead, a lot of bugs (and it didn't help that a slowly disintegrating company left less time to fix those bugs "until next release"), and any changes to the cluster while it's being used heavily leads to instability/unavailability. And many of its features were not implemented great; the SOLR functionality is a pig, for example, and "enterprise" replication is flaky.
On our team, we had a few engineers who could write Erlang, but they slowly moved to other teams, so troubleshooting production issues became a black box of pain. I'm so glad the product we had using it was sunset.
All that being said, I do not blame Erlang for Riak. I blame Basho. Their support was clueless, and their platform simply didn't perform as advertised.