Every database is an operational nightmare. Just wait until MySQL segfaults every time it gets more than 4 SSL connections at once, or takes four hours to warm up its inno cache. Or Cassandra compaction locks up a node so hard its peers think its down, turning your cluster into a recursive tire fire of GC. Or a Riak node stops merging bitcask and consumes all your disk in a matter of hours. Or Mongo ...
My point is that every database comes with terrifying failure modes. There is no magic bullet yet. You have to pick the system which fits your data and application.