It's not like VMFS (not a cluster filesystem), for Proxmox+iSCSI you get a large LVM PV that gets sliced up into volumes for your VMs. All of your Proxmox nodes are connected to that same LVM PV and you can live migrate your VMs around all you wish, have HA policies so if a node dies its VMs start up right away on a surviving node, etc.

You lose snapshots (but can have your SAN doing snaps, of course) and a few other small things I can't recall right now, but overall it works great. Have had zero troubles.