I feel there isnt really any need or place for a flash. Flash's can just mask a problem eg. Dental or physio. Why people think strapping mouths shut helps I do not know but a well schooled horse is one that is supple and relaxed. Surely it cannot be relaxed with its mouth strapped up?
There are many bits out there nowadays such as correction bits that have ideal mouth pieces for horses that struggle with normal ones. I would say a lot of the time it is the mouth piece itself as some of these breeds with fatter tongues really cannot cope with a lot of metal in their mouths.