The grey card method is prone to mistakes mainly due to the fact that you have to keep it a 45 between the light source and your subject, and you don't have to project your shadow or the camera shadow over it (very easy to project a camera shadow), and that you should avoid flare on the card, and that, I suppose, also on the vertical plane the card should be at 45 between light source and subject.

You basically need three or four arms. It's extremely unlikely you have more than 2 arms.

A grey card is a cheap substitute or an emergency substitute for an incident light meter.

Nowadays one can find relatively cheap and good incident light meters so grey cards are something that I learned to disregard since years.