You should 100% be able to share between multiple nodes. We share NFS shares between two different clusters. The benefit of NFS is that file locks and write conflicts are solved by the sharing VM as opposed to treating it as two writes to the same block device. The abstraction layer gives you this capability just like any other network share protocol.Well, you can't share the same nfs share between 2 non-cluster nodes !
imagine that you have 2 vms with same id 100, you'll mix drives with same id, like vm-100-disk-0.raw for example.
same for vzdump backup.
only for iso, lxc templates it shouldn't be a problem