I think the sweet spot of Cassandra and Solr should be mentioned in this discussion. Cassandra is more scalable/clusterable than an RDBMS, without losing all of the structure that is desirable in an RDBMS.
In contrast, if you use a full document store such as MongoDB, you lose some of the abilities to know what is in your schema. DataStax markets a platform that combines Cassandra (as a distributed replacement for an RDBMS) that is integrated with Solr so that records in managed in Cassandra are indexed and up-to-date. If your real problem with an RDBMS is the lack of scaling, but you like the ability to specify columnar structure explicitly, then this combination might be a good fit. Now, MongoDB is also a strong alternative to an RDBMS. The other thing to recall though is that the power of sharding has reached into the databases themselves, and databases such as PostgreSQL can operate with some tables sharded and other tables duplicated. See https://pgdash.io/blog/postgres-11-sharding.html.
