Depends how they've are used. If your horse is very responsive to one with light aids then that is great but if you have to be rough with it to get your aids across then it can be just as dangerous and painful as a bit. Both used correctly and incorrectly can be good and bad! I would Ben especially careful with a Mechanical hackamore as they'd provide a lot of leverage on the horses nose and it's easy to apply to much pressure.
I used one for my old mare. She has no need for bits because she was so sensitive and only needed soft suggestions for movement. I personally like them, but for my next horse I may look into bits. I'm not sure what he/she will need.