You should find it works well on both short hair and long hair. However, Magic Crystal should not be used on very stiff hair like beards and moustaches. You should also avoid using Magic Crystal on parts where the skin is particularly delicate or sensitive.
This is because of the need to use very gently pressure that moves the hair around without moving the skin. Magic Crystal works by gradually wearing down the surface layer of the hair, using a process called micro-cuticle-abrasion.
Longer hair can be more satisfying to remove with Magic Crystal as you are better able to see the hair moving under the surface and notice the point at which it begins to clump up as fall away.
It will be just as effective on short hair. Just be extra careful to be gentle as there will obviously be less hair between the removal surface and your skin.