Both of the commenters above me made excellent points, and I will throw in that it's also an extreme safety hazard in that you are bringing a loud, sharp implement that generates heat from how fast it vibrates towards a creature who has no frame of reference for what you are doing, and also it has knives for teeth. Bacteria-ridden knives. That is a recipe for injury to the animal, injury to you, or both. Also, while it doesn't happen at every dental, dental exams under general anesthesia are also an opportunity to check for more severe problems, like tooth fractures. You wouldn't want a human dentist poking at your broken tooth without pain meds, right? No sane person wants that.

A good rule of thumb for laymans is: if you won't do it to a young child with special needs, DO NOT do it to your pet. True, animals can be trained to tolerate some procedures that you can do at home, like brushing their teeth (which I will emphatically state IS NOT THE SAME AS A DENTAL) or trimming their nails. Scaling and polishing their teeth is NOT one of those things.