Around 2008 every time I went in to the dentist she found cavities. I moved states between a bad diagnosis and getting any fillings and when I went to a new dentist I came out with a clean bill of health.
I moved back and had a dentist that I trusted, she sold the practice and the new dentist started finding cavities every single visit. I switched dentists and magically those cavities were gone.
Not only did I likely get drilled and filled superfluously, the first dentists fillings fell out or broke and needed to be replaced after a few years.
If a machine was doing the work, economies of scale also create a large enough class that if the treatment is not effective the class can sue. Against a one-off terrible dentist with a small sample size, the chance of compensation for terrible work is almost non-existent.
I've recently listened to a podcast of a dentist turning consultant to increase numbers. Dude literally applied a lot of marketing tricks, tested them and analyzed the results. He did things like naming teeth cleaning, a "clinical teeth cleaning" or something along those lines. This resulted in an increase in cleanings. I guess he could be lying as he is now selling his services but the whole podcast felt disgusting.
I've had a dentist in Europe who laughed and said there's no need to come here every year, that's an American thing.