What's the difference of using nfs/efs as PersistentVolume and as a normal volume in kubernetes/k8s StatefulSet?
Use of SAN and network filesystems like EFS is not recommended for Cassandra in general, due to performance concerns. Depending on the workflows, you can get acceptable performance with ephemeral (instance) storage vs EBS, as explained here.
Adding the containers layer has not prevented those issues, at this moment we're conducting a similar Proof of Concept setting a write-intensive scenario, and we have been able to get some acceptable performance using i3 instances and ephemeral storage for the PersistentVolumes definition, but this is not a definitive response yet as we continue looking for the right configuration for our purposes.