Well, that was tested yesterday when one of my props completely broke apart in mid-air. I saw the two pieces of the prop leave the UAV from a very gentle hover at roughly 6'-8' in height and a distance of maybe 30' in front of me. I also just noticed the UAV shift in pitch for a very brief moment and immediately recover. No spiraling; no adverse problems at all. I gently brought the multicopter back to the ground and only then noticed the gravity of the break... both blades broke right at the hub.
To say that I am impressed with the DJI hardware and their programming for a lost prop would be an understatement. The many hours we might have successfully piloting a vessel can be "thrown in the air" when a critical system component fails. On a plane (as we know), this is most typically the elevator; on a multicopter it has to be a motor/prop.
That extra redundancy of the hex really shines!

Brian