There is no such thing as a guarantee in life, there are only probabilities. The goal is to make it sufficiently unlikely that data is lost, and to balance that against the cost of doing so.

That is where the disparity lies here. Reading back the data after the device reports that it has been written offers little in the way of additional assurances that it's successfully written. But if you report successful writes without syncing, there is a near certainty that you'll lose data on every power loss.