My horses have been eating out of slow feeder nest (small holes!) for years and it has never been an issue. I call bs. Show me an actual study and I might be more inclined to believe it, but all I've ever seen was anecdotal. I think we should base our decisions on science, not anecdotal evidence.
But if you want anecdotal, my 21 yr old's back teeth are worn down (incisors look perfectly normal and he has his teeth examined every 6 months). Because he's 21. Obviously, his back teeth are not affected by slow-feeder nets. Do they have to work to get the hay out? Heck yes! That's the idea! I live in Canada where the ground is covered in snow for 8 months of the year. They literally have nothing to do all day but figure out how to get hay from the haynets. I feed 6 times a day, one or two flakes at a time so they almost always have something to nibble on. This whole idea that you can just give them free choice hay doesn't work for easy keepers. They gulp it down, then have nothing to do so they develop vices. Mine are turned out 24/7 with haynets placed in different parts of the winter paddock to encourage movement. This replicates life in nature more closely since they need to move around and work to get a little bit of food in them consistently throughout the day rather than gorge themselves on 2-3 big meals.