I had one on an incisor as a teenager (a wrestling match knocked it loose and it formed an Abscess). It isn’t exactly fun, but it won’t hurt more except the needle at the beginning. I was only under local, but i hear sometimes you get the option to get laughing gas, I would take the gas if you can. It was significantly more painful before the operation than during. The vibrations are pretty intense at times and you have to stay in a pretty uncomfortable open mouth position for an hour (although they prop your mouth open for you). My mouth and tongue got very dry, and the squirts of water they provided didn’t do much to make that better. I don’t remember much of the after care but it wasn’t bad.

10years later fwiw, my root canal developed an infection and I needed dental surgery to clear it out. During this the local numbing was ineffective for some reason and the dentist had to go in 3 times to try and renumb it, but it never really worked. That really really hurt, but it is not exactly a typical case.