Retractable leashes have two major flaws: they do not comply with most leash laws, and they are dangerous to people and dogs.
Retractable leashes consist of a thin cord wound around a spring-loaded device housed inside a plastic handle. A dog on a 20-foot retractable leash has nearly unfettered freedom, defeating the primary purpose of a leash by permitting the dog to get far enough away that a situation can quickly turn dangerous. Additionally, the thin cord can break, tangle around the dog walker, or jerk the body of the dog walker when the cord is fully unwound. Furthermore, the spring-loaded mechanism has a tendency to malfunction over time, refusing to extend or retract, or unspooling at will. For these reasons, almost all of the leash laws in the United States require dog owners to use a physical leash under six feet in length.
The fact that retractable leashes are ubiquitous does not mean they are safe. Many of the things we see all the time are unsafe. Down through history, common items that were used all the time turned out to be unsafe, such as arsenic (used to make green dye), heroin (sold as a cough medicine by Bayer), asbestos (artificial snow around Christmas trees), and lead (added to gasoline).