It cards the coat and with frequent use with break off top coat, sometimes badly. I've seen dogs come in for grooming with patches where the owner has been going to town with the Furminator and the top coat is broken off and shortened because of it. It's just a clipper blade on a stick and nothing more.
IME you can get away with it on dogs with double coats that are short/not super thick. You couldn't pay me enough to use it on dog with long double coat, however. Too much potential for damage. And keep in mind, as long as you keep brushing, the hair will keep coming. When people try to brush "until the 'shedding' stops" they end up demolishing the dog's coat because they're just ripping out hair, not working dead coat. The hair will never "stop coming" when you use a Furminator.
Honestly, I find that traditional tools like an undercoat rake, slicker brusher, and good bath and forced air dryer are more effective and healthier for the coat.