Postgresql cluster over nfs
Short answer: NO!.

Longer answer: If you use synchronous NFS and you have robust STONITH / fencing, then with great caution you could run a shared-storage failover cluster over NFS without horribly corrupting your data. However, it won't do you any good for failure modes where the data is affected by the problem.

Shared-access SAS/SCSI, DRBD, shared-access SAN storage (iSCSI etc), or similar block-level sharing is slightly saner than using NFS, but still not safe.

If you don't already know what STONITH or fencing means, then see "short answer" above.

Use replication. Really.