You had me until the "like Cassandra" bit ;) If there's one thing where Cassandra loses the battle with other NoSQL tech then cluster managements is probably it. Also, it's a bit unfair to compare RDBMSs with Cassandra. Clustering is inherently going to be more complicated for RDBMSs. Incidentally it's actually where I feel MongoDB deserves a bit of credit.
It is still cumbersome to use, hard to shard, even harder to cluster and is incredibly complex to manage compared to databases like Cassandra.