Safer than sailing across!
Leaving the pax behind for a 3 engine ferry means zero risk to the pax from that operation.
Flying over populated areas on 3 rather than 4 increases the risk to people on the ground. The risk is miniscule, but it exists.
The ferry is done to get the aeroplane fixed more cheaply than if everything had to be taken to the aircraft for it to be fixed on site. The risk is accepted for a commercial reason.
Risk is balanced against cost, and this happens every day, on every flight. It would be safer for all pax if they were given high energy 'space' food in little bags, rather than have the danger of trolleys, bottles, cutlery etc loose in turbulence. It would be far safer if all seats (OK, except flight crew!) faced backwards, but pax don't like it, so commercial pressure reduces safety. How about replacing the weight of on-board entertainment systems with airbags? I could go on.
Aviation is probably the most risk-assessed and regulated human activity, and if all this assessment and regulation means that it is deemed to be permissible for a particular 4 engined aircraft to proceed on 3 with an acceptable degree of safety, and if the bloke at the sharp end agrees with that, then what is the problem?
The flight is obviously not as 'safe' on 3 as on 4, but it is a minor and acceptable risk.
IMHO, the interesting topic to discuss is not the safety of this flight, it is why the FAA reacted the way it did.