In my experience, it's three things:

1. You need really good surface contact to get the diffraction on, so you can only really apply it to flat, uncurved surfaces

2. You need to apply it while the chocolate is hardening. If you're using a mold, you cast the chocolate upside-down so only the base is exposed during setting. You can't diffract the part people will actually see.

3. The effect is really fragile. Chocolate melts at around body temperature, so if you hold the piece for too long, the diffraction disappears. I've been able to transport diffractive chocolate in a padded box but don't think I could wrap the chocolate directly.