You don't hear about them because they don't work. There are a lot of dishonest scammers who prey upon people desperate to 'cure' their stutter using tricks and gimmicks who will gladly lie and cheat people out of their money for devices that don't really work. All stuttering devices, including DAF, appear to help for a short period of time. Then you become normalised to them and your stutter comes back even when using the devices.