As for prongs, I would use one on a BIG (like mastiff or big AB or even a very strong APBT) dog who was STRONG on the leash first and foremost so I would have control over the dog during the training process. I wouldn't use the prong to actually correct the dog, just so I could hang on to him while leash training with the goal of eventually moving to a martinagle-type collar or flat collar and leash. Mastiffs and bulldog types are pretty hardy, and no-pull harnesses don't always work well on them (I know this from experience at FOHA), and if the dog happens to be hairy, like a Saint or a Newfie or something, all that coat actually protects the neck pretty good. I know some like the "be a tree" and the turn around when there is pressure on the leash methods (I like those too), but "be a tree" doesn't work well when you have a dog who can pull you off your feet with ease. As for turning around, put your leash and collar on a tree, then try to turn around. That's what its like with big, really strong dogs.
I've used a prong. It didn't really work on the dog I was using it on, because she'd pull on it hard enough to choke herself, but I don't feel bad about using it
The thing is, no piece of equipment is really perfect. I've used front clip harnesses, too. Tess (the dog I used the prong on) could houdini her way out of a front clip. The front clip was really effective on my Tully,
but it worked largely by inhibiting her front end movement, and when you're walking for distances (exercising the dog), I don't think having the dog's free movement compromised is in the dog's best interests.
I dislike the head halters because of the risk to a dog's neck. I did purchase one version called a "Canny Collar", which has a loop over the snout, but attaches behind the head, and should be safer. However, Tess can houdini out of that, too.
In terms of safety, I think the prong is actually safer than the other management options. The best way to deal with pulling is to actually train the dog, but that can be easier said than done. I'm not going to criticize someone who finds a way to manage their dog that works for them, and the dog is happy and healthy.