I think his comment about stateful sets is more focused about running data stores on distributed filesystems, or network attached storage. This isn't a good idea generally, and I am not going to go into why, but Cassandra and Etcd advise against this for a reason, its well documented. It wouldn't just be using a 'better' storage system, it would be designing and implementing a very complex operator as he mentions.
The real take away which I think we should all realise is that, this is shiny tech to run mission critical data stores on, unless you know what your doing and want to invest a lot of time and effort in to it. Right now it's probably better to use RDS/Cloud SQL, or just run a traditional postgres setup with a SAN and decent failover that is tried and tested.