Yes, the grandsons worst concussion was from a terrible sling tackle.Concussion is likely a concern for parents regarding all three contact sports here in Australia, and the AFL for obvious reasons is essentially outlawing the sling tackle, especially the holding the arms and slinging motion.
I have also seen a few players concussed or suffering facial fractures from a raised knee. Yes raising a knee, whilst jumping for a mark, to gain height and clear space has always been allowable, and provides for some spectacular marks, but i am not too keen on players standing under the ball and having a knee thrust into the back of their skull, but on the flip side, if raising a knee whilst jumping for a mark was eliminated is it taking too much away from the fabric of the game?
The knees up in marking contests is a tricky one, my gut tells me it is an accepted risk when you decide to play the game.