I fully agree with you that a special luminance meter, if you have decent one, is more precise and a lot easier to use than a camera. And you have to know beforehand how a certain number of nits translates to camera settings. ( I have a luminance meter myself for calibrating my monitor.)
If you use a camera for this purpose you need to have a decent photographic knowledge how to measure and in what circumstances. As I argued, the distance does not matter but the ambient lighting does, so the camera measurement should be taken in a fairly dark room. This proces is highly repeatable because camera's and lenses are very constant in this respect.
I agree it is a lot of work especially for a less advanced photographer, but you will save a lot of money for an instrument that you only use a few times.
If you are a reviewer of monitors, brightness, etc. then you need a luminance meter. However if you are a user, a camera can do the job
Ton