Actually the hard part of teaching is the fact that just about every examiner has his/her biases regarding ski technique. We have seen many MA threads where there are as many ideas and suggestions as there are posters.
Still, the experience acquired over the years is very valuable. It allow some instructors to have a far better feel for MA, fixes and progressions than I ever could have. Not to mention that I follow a different teaching method, so my suggestions would different anyway.
But I do agree with you, teaching effectively is an art and hence difficult to master.