The replication is synchronous but not the cache. Replication of the cache on some technologies is very expensive $15k+++.

NFS or the NAS Auto Failover is planned for this year - most likely later part. This is very complicated to do so we need more time.

WB
Write-back cache. This functionality improves data writing performance. Writing is considered completed as soon as the data is stored in the disk cache. Later the disk cache commits the data to disk.

Note

Please note enabling write-back cache may cause data corruption in case of a system crash (e.g. after power failure). Corruption may also appear during failover processing. We strongly recommend using write-through cache if you are transmitting very important data.

If RO and WB are disabled (RO and WB are unchecked) then Write-through cache is used by default. This means that data is not stored in cache. Instead, all data is considered written after it is committed to disk.
Also, how's NAS (NFS) autofailover coming?