I don't know if your post was sarcastic or not, but it sure sounded like it, so here's my reply... If it wasn't, just ignore me.
The one I saw was a 15lb sack and it was from about 10' above their heads. The balance beam was about 8-10"s wide. Not that hard.
Have you ever actually tried something like that? I have, and as long as you're light on your feet and know how to step, they're not hard, either... Especially only 4 of them. I watched 2 hours of the show and saw 2 people fall from it. Granted, they didn't show everyone that did it, but they showed more people fail on that oscillating platform than on the balls.
I didn't see that event, must've been on a different stage, but that does sound ridiculous. I know there was some crazy lookin' stuff on the second stage, I can't imagine anything past that point.