sync=disabled basically tells FreeNAS to do unprotected write caching. In short, when a write request comes in from a host or VM, FreeNAS with sync=disabled will write it to RAM and acknowledge write success back to the host or VM immediately without committing it to disk or some kind of permanent storage. Most of the time, this is safe for home labs with a UPS, but not recommended for business or datacenters.

Imagine you home loses power and your UPS runs out of juice. You'll most likely get data corruption because of the uncommitted write transactions.