Police dogs, armed forces dogs and assistance dogs may also not be trained using electric shock training devices. (thekennelclub.org.uk)
With shock training, some dogs may learn to fear people, objects, or situations they associate with the collar. (caninejournal.com)
In terms of other alternatives, Top Dog Tips has previously publish a great piece on this, listing all possible options to training canines for those who choose not to use shock training collars for dogs — No More Pain: Best Alternatives to Dog Shock Collars. (topdogtips.com)