“I guess they drew the line at getting their legs broken.” The volunteers were bound at the wrist, not nailed. And the situation of broken legs was simulated by the volunteers by hanging only by the wrists — they could still breathe just fine.
I’d think that actually getting whipped and nailed with the pain and inflammation and at least the beginning of infections that accompanied, might create a different situation of vulnerability. Expert takes on this differ in details, but the Romans made sure it was nasty enough to be thoroughly dreaded.