aside from getting caught in fencing like in the States, most (near) fatal accidents seem to be blows to the head. Surtees, Wilson and (almost) Massa were the direct result of having no form of deflection. Bianchi and De Villota were killed by hitting objects that shouldn't be near a racing car.
A halo solution like Mercedes rendered could be integrated into the chassis with a quick release system just like the headrests (but a little more sturdy). It would just take seconds, and when a driver is unconscious after a big crash, it will take at lease minutes to extract him out of the car with the back supported chair anyway. The last time in F1 this was necessary, it was for Massa, because he was struck in the head.... the last time a driver was upside down and on fire was in the 80-ies at least.
I used to have a friend that thought seat-belts were unsafe, because he knew someone that was thrown from a car and that had saved him.
Mercedes has designed their solution to go up, a system where you (after you pull out a pin) slides off forward, just 5 cm before it detaches, could be a simple solution.
No real life solution will help crashing into a truck at high speed or a sharp thin blade like De Villota.