Flashbang is a short-range solution, and not a very effective one at that (one tile walking to be able to throw around corner, pulling it out of holster, and throwing it is more TU than a single shot with any plasma weapon, which sucks when up against blaster, as one ball is enough to wipe troops.
However, this effect would be too extreme with needlers, and maybe even machine guns. It is one thing to maybe force a heavily armed taman to not be able to aim shot a plasma rifle, or not be able to do a massive spray & pray back with a needler of his own, but when you can just calmly walk a troop out of cover through an effectively stunned alien's FoV, it becomes too much.
I propose a different thing. Every TBD (probably 5+) shots landing within one or two tiles of on actor cause the spread of its weapon to increase by a good amount (~0.7) for one turn. A potential feature to balance this could be if the actor runs several tiles away, this effect is removed.