That's what I've seen. I've been involved with efforts to do something that an actual RDBMS is designed for with Mongo, Cassandra, Couchbase, Redis and even ElasticSearch - in every case we ended up re-creating so much of the functionality that is built into an RDBMS that the effort took much longer than it would have taken if we'd used MySQL or Postgres, in addition to being much slower (not to say anything of how proprietary and locked in the solution ended up being). There are good use cases for all of these products, and when used the way they were designed to be used, they're great; they're just not used that way very often (that I've seen).