If you are having difficulty understanding a problem, try drawing a picture.

the method of drawing a picture is pretty well understood, thus understanding the limits of the method also are.

However understanding the limits of using body language as a lie detector in any instance where used requires understanding the biases of the person doing the detecting (which may be yourself, and knowing one's self is traditionally a tricky thing), what their current mental state is in, the culture that the person who may be lying is from, do they have any ongoing problems or conditions that might make them appear to be lying when not, is the potential liar able to mask their lying better than most people, and probably a few more things that are hard to pin down that I haven't thought of here.

Getting the heuristic to work in reasonable, measurable way so that one can decide is this giving a good approximation of a correct answer, am I making progress using this, is an important part of having an heuristic. As an example, common heuristics used in construction or software project management.

So I would think heuristics that are well understood and that do not depend on imponderable questions that philosophers have been arguing about for millennia should be used, but ones that are not free of these problems should not.