Skiing rapes our knees when you fall because your legs are separated, and this makes them a lot more vulnerable to being torqued, and as a result tearing ligaments and other injuries, whereas on a snowboard you're feet are attached to one board, so generally when you fall, you're knees don't get torqued as easy unless you're board snaps or something. Knees take a lot more damage from falling skiing, but I'm not sure about other aspects, like cliff hucking or skiing bumps.