My old boss told me in 2019 to stop using clippers on the neckline and I finally get it | Barbers - Freelance Forum - The Community for Independent Professionals
Clippers on the neckline always felt fine to me honestly. Maybe your old boss had a bad experience or something but a clean line is a clean line. Been doing my own neck for years with a trimmer and nobody has ever said a word about it. Sounds like one of those rules that works for some hair types and not others.