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Ye I think you're advised to have a pad behind the head rest to limit the total amount of flex. We've had them without that though in the passenger seat of a Caterham and never had any issues with strength so I reckon they're pretty solid. We're about to fit a cage and I feel like I need to compensate for the extra weight!
The only problem I see with them is they are designed for cars where the car pretty much wraps around the seats ie Caterham Westfield etc. In the event of a crash in one of those the harness limits your frontal movement and if the seat were to move under compression you are only going to go a few cm's until you are supported by the car.
In a Clio though if you were to have compression (the opposite to what the harnesses support) and the seat broke you would go backwards toward the rear seat area with potentially dangerous consequences