I suspect that the real risk source from tablets and laptops is that if the plane moved violently they would be thrown around the cabin and they are heavy, hard, and a bit pointy.
I dropped an ipad onto a hardwood floor once from 2 feet up. It hit straight on one corner and took a 1/4" deep chunk out of the floor (ipad lived, weakly attached magnetic cover was judged stupid).
Phones, eh, probably light enough to be unlikely to really hurt someone, but an ipad to the head could really mess you up.
