Go back and look at your pic in post #6, the disc fitting forms a rotational pattern of a more oval shape then a true circular one, looking at it 70% of the magnets likely fail to be pick up by the stationary sensor head.
It is the diyer who doesn't understand or is familiar with being practical at fitting given that instructions are available to do the job correctly, we have seen it all before.
Your disappointment should be the fact that one has failed to install one critical small part correctly and it is the fitter and nit the kit at fault. Once rectified and fitted correctly one will find the kit will work fine, once fitted correctly ensure the gap from magnet face to sensor head is approx 1mm and not much greater.