Does not work optimally on many sewing machines

The idea of ​​such Saumhilfe is really good.
With most machines, the needle plate's metallic and magnetic. However, the case is usually not magnetic (although metal).
The Saumhilfe is a fairly large standard magnet powered (no Neodymium), which just gives an OK, but dislodgement during sewing excludes no means.
Moreover, these magnetic region must lie completely over the needle plate, and can thus be so only narrow hems lead (about 1cm).