if you have more than two ESXi ( = >3), you can use VSA to create shared storage device which is low cost. because I have noticed that some time the VSA management network and SCSI network losing network connection for few minutes and establishing back. this could cause unsync in cluster. let say if your having 2 ESXI and you want to do patching for one of them and u would need to power off one ESXI based VSA and another one VSA will be in up and running. in this moment if another one loss network connection you will loose shared data store connection lost.
What I would like to know is if we can use a combination of local storage (storage local to ESXi hosts) and a shared storage (like NFS or NAS or SAN).
yes you can but this SVA SAN app is used primarily used for Local storage sync rather than NFS or NAS or SAN.