Personal experience from using every night for about a year.
It hurt at first, but over time you start feeling that gum pain less. Now i basically don't notice it. I pretty much only use the standard orthodontic type (my main is the littleforbig bigshield v2), and since I started and have gone to the dentist a few times, my teeth have not gotten worse. On the contrary, my teeth have actually gotten BETTER since I started, making my teeth straighter and helping close the gap between my front upper teeth which had been present for about a decade (likely from making the gums more malleable combined with moving the teeth around every day?)
If normal pacis could mess up one's teeth, I would be an example of that by now.