The purpose of a dog leash is to keep your dog safely under your control, but the retractable style of dog leash has come under a lot of criticism for inadequate construction and allowing dogs so much freedom of movement that it can put the dog at risk.
Retractable leashes can provide your dog with a lot of freedom of movement (some leashes extend well over 20-feet) but that distance means that your dog is not within your immediate control.
There have been many reports of dogs walking out ahead of their handlers on sidewalks or along the side of roads, and having enough freedom of movement where they can turn and dart into traffic, or make uninvited contact with people, cyclists or other dogs.
Certainly when there is a threat or emergency, having your dog 20+ feet away from you prevents you from immediatey managing your dog as to keep it safe from immediate threats or being a threat to others. The time it takes to reel in a dog that is at the end of a retractable leash could be the difference between difusing a potential situation before it starts, and having to deal with the consequences of an incident after it has happened.
Another issue pertaining to a long leash is the risk of the leash tangling around objects, the dog winding itself around objects, and even wrapping around the legs of the handler or the dog itself. An extreme example, is of a man who had his thumb amputated after the leash cord accidentally wrapped around his thumb before it went tight.
Most retractable leashes have little more than a thin cord, and there have been numerous reports of these leashes breaking from the impact of a dog running to the end of the leash length.
When you think about it, having 20+ feet of leash length gives a running dog more than enough room to get up to full speed before it reaches the end of the leash. The resulting impact can snap the cord, detach the cord from the handle, or even jar the handle straight out of the hand of the person walking the dog.
Additionally, retractable devices are prone to malfunction over time which, in the case of the leash, might mean failing to extend or retract, or even unwinding without any control.
For dogs that don't already have adequate leash-manners, retractable leashes can reinforce a dog for pulling because the leash can extend when the dog pulls on it.
Are They All Bad?
There is always two sides to every story, and retractable leashes can indeed be a useful tool when used properly and in the right situations and environments.
They are best suited for dogs that already have adequate leash-manners, and should be used in areas where there isn't a risk of the dog getting involved with vehicle traffic or interfering with pedestrians, cyclists, other dogs, etc. Regardless, always read the safety instructions that are provided when you purchase a retractable leash.