Hi Andy,
I have checked the user manual again (chapter 39.1.2), I think the exact word used was "not recommended" because it is "NFS is slow" , as shown below:
This style of high availability differs from data replication in that it requires a shared file system which is accessible by both the live and backup nodes. Typically this will be some kind of high performance Storage Area Network (SAN). We do not recommend you use Network Attached Storage (NAS), e.g. NFS mounts to store any shared journal (NFS is slow).
Based on your answer, I suppose, SAN is the only way to store the shared journal?