I posted this a while back
With a shield booster, a child is sitting in a booster, thus, boosted up. The shield is in front of them, but by no means snugly against them as the sole restraint. The belt goes over the shield. The problems with this include being up higher - different center of gravity, and not having anything snugly against the bony parts of the body like you would with a regular seat belt, booster or harnessed seat. If there are no head rests, you've boosted a child up with nothing behind their head. Also, the chances of ejection, either out the top or down out the bottom, depending on the size of the child and the type of crash, are high.
With the LapTop, the child is sitting right on the vehicle seat and the LapTop sits over their legs and against their chest. It is snug against the bony chest and the child is lower down in the vehicle. With a shield booster, the shield is the restraint - the shield is there to be impacted by a body. The same applies to an overhead-shield convertible seat - made to be impacted by a child's face or chest . Since the LapTop is already snug, like a harness, there's not the same kind of impact.