Any tool can hurt the dog when used improperly. Flat collars, chains, and martingales can put a lot of pressure on the trachea and collapse it or do major damage to it even when used properly.
When used properly, and prong collar does not hurt the dog or risk collapsing the trachea.
I use prong collars because I cannot physically control some dogs (without going mad) on a flat collar, martingale, etc. and do not feel comfortable with the health risks in using head collars, front clip harnesses, etc. It's basically power steering while I use positive reinforcement to train the dog how to walk nicely on a flat collar.
Danielle, owner of...
Fraggle Rock Collars & ...
Fraggle Vom Richcreek CGC CA
Red Dog's Reconnaissance Man CGC CL1-R CL1-H CL1-F CL1-S USJ
General Patton Vom Winter Storm