"The Meguiars Mirror Glaze says "for professional use only", why is that?"

Mark, you would have to ask Meguiars. Polishing cars is an art. and can easily cause issues, especially with power buffers. This glaze is easy to apply, but for the best results, needs a certain level of experience. I don't use power buffer. I've never had the need, and having done the concours circuit for many years, I developed a technique which can clean a car after 100 plus mile drive, within the 60-90 minutes allowed at an outdoor event. So in some ways I guess I'm a professional polisher anyway!. The Morgan was done in around 30 minutes, and I did my Golf, which took a bit longer because there's more bodywork. maybe around 40 mins.

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4