There are so many variables, and like you say, the guesswork to determine the frame and the body part are so basic. The Chelsea offside and ours are to close to call, inconclusive.
Been a lot of talk about frame rates, which is 100% a problem because the maths simply doesn't support the level of accuracy they're applying.
But there's also an issue of camera angles, they're essentially drawing lines based on guesswork as the camera is rarely dead in line. Posted this before, and it relates to hockey, but it illustrates how much the camera not being in line can affect things.
In simple terms it is just not possible to use the tech in the way they are trying to use it, the margin of error is so significant you really need to go back to 'clear and obvious', something that you can see on first viewing at full speed. Not something that takes 5 minutes of slow motion and drawing lines.