Why is `ZFS over iSCSI` clones support limited?

[Giovanni]

Why are clones not supported when using `ZFS over iSCSI`? Refer to table in: https://pve.proxmox.com/wiki/Storage:_ZFS_over_ISCSI

In contrast, NFS has support for clones via qcow2. https://pve.proxmox.com/wiki/Storage:_NFS