I think they are bad if they’re misused. If you brush your dog with all your strength for hours on end, you’re going to seriously damage their coat. I think they are fine with moderation for any double-coated breed, so your labrador should be fine. Talk to a groomer who can show you in person how to use it, or watch the many, many YouTube tutorials.
I have a GSD mix and I use an undercoat rake/slicker brush that I bought from PetSmart like 10 years ago (but here’s the equivalent that you can buy today). I brush my dog about twice a week or whenever I think about it, but much more often when she’s blowing her coat. I know a lot of owners of double-coated dogs do something similar, either an undercoat rake or a greyhound comb and then a slicker brush. It definitely takes more time than the Furminator, but all in all is safer imo.
-C