The term dealer mechanics used to use was “the fresh air and sunshine treatment”. I think this was more common in the past than it is today. I rarely use a dealer but when I do, I check their work after and haven’t yet found a case where they didn’t do what was promised.
But my mother had issues with a dealer back in the 70’s. She bought a 71 Capri and every time she took it to the dealer, it came back running badly, idling badly mostly. Every time I found they had adjusted the carburetor, I would readjust it for her. I knew they did this because they had to remove the plug the first time they did it.
When it had about 20k on it, she called me and asked me to look at it. The dealer told her is had a bad spark plug wire and she needed a new set. I checked and sure enough it had a bad wire, they didn’t even bother to match the boot color on the bad wire they installed. She worked for the company that held their insurance policy, so she told her boss. She got special treatment after that.
Fast forward, she buys a 78 Zepher Z7 from them. She takes it in regularly for service, but they haven’t put the new car to her so she is no longer getting the special treatment that she thinks she is getting. I’ve been stationed far form home for several years and eventually I get in a visit. She is complaining about how hot the shift stick (automatic) is getting. I check, the ATF is clear as water. The manual calls for a change every 20k miles, it has 40k on it and the last service receipt show that the ATF has been changed. She gets it changed by a mechanic we know that has a good reputation and no more problems, and no more business with this dealer.