If you drive somebody's entire body weight through their head and neck, they're in for a bad time.
The first and most obvious injury is severe head trauma. A depressed skull fracture will put a load of pressure on that squishy grey matter in there, potentially leading to haemorrhage, pressure build up and even something called brain herniation, which occurs when the pressure inside the skull is so high that a bit of the brain pops out just like an abdominal hernia. If this is accompanied by an open wound, you risk directly exposing your brain to infection from the outside world.
The skull is reasonably sturdy, so there's a chance it might survive the impact. Your neck, not so much. Here we have the risk of spinal injury, compression fractures and even our old friend cervical dislocation (of curbstomp fame, remember?)
The fact that this kind of move brokeStone Cold's neck and temporarily paralysed him,even when supposedly simulated tells you all you need to know.