IIUC the problem is that LVM snapshots consider only the host you're taking 'em from.
That's no problem if you have iSCSI as shared storage and have only one VM accessing the LV, as long as you take the snapshot from the same host where the VM is running.
But if you have an iSCSI LUN shared by two VMs on different hosts, and those VMs use a shared fs, when you take a snapshot from host A, writes done by host B go undetected (no COW in the snapshot when host B writes = corrupted fs!).
So, if you need to be able to take snapshots of a LUN shared between multiple active VMs you have to be sure your iSCSI server allows that!
Probably, such a server would prefer to export single LUNs, one for each virtual disk. IIRC (I used 'em 6 or 7 years ago...) EMC2 controllers did it this way.
