Modern shock collars are NOT what most people think they are. I get why people don't like them, the idea of them, whatever, but the fact is they are not even close to what the vision of them are. People have this mental vision of shock therapy from the old sanitatarium days and they transfer that directly to shock collars. That's just not the case.
One is direct skin contact with electricity, REAL, LIVE electricity, similar but lesser in voltage to the electric chair. I'm pretty sure at one time, this was achieved by sticking bare electrodes under the eyeballs.
The other is electrical stimulation with VERY low voltage from an onboard battery, a battery that needs to be charged after 17-20 hours, EVERY 17-20 hours or its dead, flat dead. Use your brain, just how much "shock" do you think you'll get from a battery carrying that amount of electricity? The batteries in your tv remote carry more pop than a shock collar.
The fact is it's similar in delivery to what you get in a doctors office while recieving elctrical stimulation on muscles. And that is direct muscle stimulation, to make whatever muscle contract, either rapid pulse contractions or sustained (10-15 seconds). Hell, you can buy lower grade muscle stim sets straight up without the doctor being involved. They don't cause pain. AT WORST, it can be unpleasant, as in an unpleasant sensation.
It's called a training collar for a reason. Whether you want to acknowledge it or not, the "shock" from a shock collar should be understood in the same context as being "shocked" by a loud noise or something equally "shocking" as in surprise. The purpose, and the point, isn't pain, it's to get attention, IMMEDIATELY.
You have BIG HEAVY BULKY dogs that WILL drag your ass to whatever they want to reach in certain circumstances, and they WILL flat out act as if you do not exist because they are INTENTLY focused on whatever has their attention or ire or whatever. This situation can go very badly for whatever or whoever the dog focused on, badly for the dog, badly for you. Depending on the dog breed and it's size and strength, things can go south real quick. It doesn't matter how well behaved your dog is, how well you think you know your dog, how great you THINK your relationship is with the dog. Individual dogs are, like humans, individual personalities, every dog is different. Situations like those are what a shock collar is made to handle, or, if your lucky, prevent. These are for getting the dog's attention right now. Because if you don't break the dog's focus for a moment, the end result might be very bad for all concerned.
There will be exceptions to every rule, but, most dogs, one low intensity "shock" is enough. Use the beeper or the vibrate fucnction, and THEN the shock. NOW, the dog knows what the sound and or sensation means and they don't want the shock, it gets the attention RIGHT NOW. ALL you have to do is break that focus and YOUR dog is now fully present and you can direct the dog to do whatever. Ideally, this will have happened BEFORE you run into the situation that's potentially very dangerous, so you can prevent the situation before it really starts.
On the street, Rottweilers are very people friendly, but HYPER aggressive toward other animals. Pitbulls are SUPER territorial. NOT all dogs in all situations, every dog is different, but those are KNOWN traits for those breeds, other breeds have the same or similar traits. A responsible owner knows his breed and knows how to deal with the issues those breeds have in regards to most things. It boils down to you, as the owner, are you prepared to handle the situation or are you just going to leave it to chance?
You may think it's cute or funny that your loving attention whore Pitbull goes apeshit when the mailman steps on your porch, because he just goes all rigid and barks and growls up a storm but never moves to attack, but, I promise you, one day, with the right conditions, that dog is going THROUGH your door for that mailman. Then you have a big problem, and most likely, a dead dog on your hands and a lawsuit you are going to lose. And it can get worse from there, depending on the situation.
Modern shock collars are a training aid and little more. The problem with them, if there are problems, come from humans being who humans are.