This is what it looks like. It does get dirty, including the front lens of the light. For optimal flashlight performance I'd clean the front lens every 200 rounds or so.
No other issues. It's actually beneficial since there's no part of the gun obscuring the flashlight's beam.
One aside is that it's quite a heavy fore end being bulky and solid plastic, especially since the weight is out front. I'm thinking of switching to a Magpul fore end, and adding a rail for a Surefire x300 pistol light. The shotgun specific fore end light has better implementation though.