How do KFm step airfoils work?
The idea works but only with active automated re-configuration of the
  shape of the step(s) during flight. Further testing has shown that
  this airfoil is effective in low Reynolds number flow.


source: Wikipedia

The Reynolds number is indicative of the speed of the airflow over the wing. The higher the speed, the higher the Reynolds number and vice versa. This is why they would be more suited for model aircraft rather than large aircraft which would experience higher Reynolds numbers.