That will basically pick up every piece of electronic equipment in the room. Given that cameras can be hidden in them too, and that you'd have to take apart each one to be sure, I don't think a NLJD will tell you much.
They were developed at a time when electronic equipment was far less common, so the presence of electronics was more suspicious; now, you can come across things like deeply embedded RFID tags (only used for inventory tracking) in otherwise completely non-electronic products.
You are correct, they pick up a lot. But depending on the type of engagement, you just end up taking apart every single thing with a circuit in it anyway.
There are defeats for nonlinear detectors, like a three port circulator with a resistor. In turn there are more advanced detection methods, but now you are getting in to tools not publicly available.
They were developed at a time when electronic equipment was far less common, so the presence of electronics was more suspicious; now, you can come across things like deeply embedded RFID tags (only used for inventory tracking) in otherwise completely non-electronic products.