I'm curious how much firsthand knowledge you have of these systems.
1) The electrical shock is not truly painful. It's startling and highly unpleasant, but it's not painful, for short bursts. I can't speak for all systems, but for the ones I tested (albeit holding the collar in my hand, not on my neck), it was no worse than typical electric fencing, which is widely used.
2) These systems have the benefit of not affecting other animals, and dogs and other animals cannot get stuck in them. We had an electric fence around a duck nesting ground growing up, and I'll never forget the morning we found a whimpering fox that had somehow gotten tangled in the wires. I guess it had jumped at the shock and pulled a wire off the fence? I don't know. But it had the electrified wire wrapped around a back paw. At least it was a pulsing fence (rather than constant current); so the fox didn't appear to have a bad burn, though I'm sure being stuck in it for hours was terrible (and truly painful).
2b) They also do not impede the traffic or cut off travel corridors of other wildlife, which can be a real problem in some areas.
3) Ordinary fences are (often) less effective. Given enough time and a large amount of fence that cannot be thoroughly checked & repaired regularly, dogs will find a way to escape. They'll dig under, they'll jump over, they'll climb, they'll chew holes in, etc. We had a super smart lab who figured out how to climb a chain link fence. Depending on the risks of escape, this can be highly dangerous.
4) The risk of shock is typically very low. When these systems are installed, they are typically buried along natural boundaries, and when they aren't, you put temporary flags in the ground to show the dog where the boundary is. So the dog very quickly learns the boundary, learns what the beep means, and rarely gets a shock after the first couple of days.
Overall, I don't think these systems are signficantly less humane than other types of fencing – probably more humane if you're comparing them to traditional electric fencing.
That said, these fences aren't perfect either. I have another friend who installed one to keep his husky from leaving his ~3 acre yard, because the dog was breaking into a neighbor's chicken coop. It worked for a while, but eventually that husky realized she could just get a running start, jump over the boundary, take the brief shock, and then be free. The drive to get those chickens was just too compelling. Of course, the drive to return home was not quite as strong. So she'd sit at the edge of boundary, whining to be let back into the yard.
A traditional fence might have been more effective in that case, though I think that husky would have found a way out. She was relentless and clever. They had to get rid of her eventually, when their neighbor threatened to put out bowls of antifreeze – now that's disgusting! (To be fair, this was in rural Alabama ~15 years go, and that neighbor depended on the chickens/eggs for food. Still disgusting, but the frustration is understandable if a dog is literally taking food out of your kid's mouth.)