1. I've done them heavier, and my back is fucked up. The guys at Westside don't care if their backs are fucked up, and they like them so they probably work. They worked for me, but like I said, my back is fucked up. RDLs are safer and can be trained at the same intensity without as much spinal damage.
2. Make sure you're not trying to do them with completely stiff knees. Unlock them a little at the top before you drop the bar down.
3. Of the 3, I think RDLs are by far the most useful.