With it mounted on the barrel or mag tube, it's hard to stay in contact with the light. Could be a big deal for some people, but for most ordinary joes I suspect that can be solved by avoiding momentary switching in general. If someone insists on it, they could always use tape style switches but that presents its own issues related to use and mounting.
With the light mounted to the forend in any other fashion besides the Surefire forend there's a higher chance that the light will get driven into your hand under recoil...especially if your recoil control techniques are not on point.
When a 12 gauge drives pointy things into your flesh it is doing so with a lot of force. It will split nails to the quick, tear gashes in your skin, etc. There. Will. Be. Blood.
You also tend to get a bigger dead spot in the light because you have more gun sitting in front of the lens of the light to block it.
JLW is smart and that Fab Defense forend seems like a pretty good solution as you can mount a light where it won't hit you.