I once had a horse that was slightly crazy and super spooky, but freaking gorgeous and one of the best movers around. Anyways, I could only use draw reins on her because if she spooked and felt the pressure of something holding her head, such as a MH, she would rear and flip, or fall over. I just think that they are dangerous, because that could happen to anyone, even if their horse wasn't quite as spooky as mine. I always like to have a training aid that I can control so I won't ride a horse in side reins or a MH because even on the loosest setting, they can still feel it.
In the end though, neither draw reins nor the MH really accomplish the job they say they do, but they can help build a topline so that when you really train them to collect, it won't be as hard.