Personally, I am not convinced by dream masks' effectiveness, for a couple of reasons. Firstly, it has been proven that the brain does not respond to external stimuli most of the time during sleep. Occasionally, people will hear a sound in real life and it will be incorporated into their dream, but only very rarely. I doubt that the light stimuli will be able to incorporate into the dream - and if it is picked up, I think it would be more likely you would just be woken up. I think it's very possible that people buy the mask, and flashing is incorporated into their dreams because they are expecting it.

Anyway, that's just my opinion, and I could definitely be wrong. Still, I think normal techniques work just as well.