Chain punching the top of the head doesn't do damage. Try it for yourself. You will feel awkward.
Biu jiing the eyes works but you have to pull back after doing a palm strike . Crane beak doesn't penetrate the top of head unless you have iron fingers. Snake hand strike after doing a palm strike which was hangin in the air wouldn't hurt the top part of the head either. Elbowing the top of the head works but you have to curve your arm. The best few moves that would be faster after the chain or palm strike that hangs in the air(and needs to move downward) is a pek chui or a kup choi. I made a mistake here by suggesting sow chui. Sow chui requires a pulling back action so it's not fast enough here.
My point is to have varied options to use at your disposal. Somebody who is unfortunate to have learnt only limited hand techniques,say pek chui and kup choi etc and not knowing chain punching, biu jee, snake hand ,crane beak and elbow is also at a disadvantage.
Hitting the top of the head?!
You do know the top of the head can take serious damage? Ever tried running into a steel bar at full speed? It hurts but it does not knock you down (yea I did get quite dizzy after a while when it sank in what I just did). Now imagine if your skull can handle a steel bar, how well would those knuckles take it?