Dumbfires only really seem to work as ambush tactics. In the video the Python goes up and rubs noses with a Viper that has its hardpoints out, that's either inexperience or hubris. If someone comes up to you without opening comms with their hardpoints out it's an extremely aggressive stance. All you need to do is check their components to see what weapons they are carrying, if you see dumbfires you can safely guess their intention. The Python has a small enough hit profile when facing another ship to make it a smaller target to hit at any range, so the Viper would need to get lucky if he didn't get the ambush (I wonder how many times he had to fail at this tactic to get the video.)
If you equip DF missiles you are vulnerable to anyone who is aware of your prescence and a smaller target will be a nightmare to hit. I fought the missile Eagles in Lave, died the first time in my Viper as I'd never encountered DFs before and stayed at close range but each time after that they lost. The tactic is simple, boost, FA off, flip around, FA off (I'm using K&M, hard to actually aim with FA off) hit reverse thrust, fire at will whilst using some lateral thrust. They'll rely on luck to hit you as the range will make it hard to predict where you'll be when they fire the shot.
I agree with the other comments about giving the DFs a hitbox, I think that's fair. The sacrifices a pilot has to make to use missiles are enough to balance them as far as I'm concerned. You have limited durability, depend on ambush tactics, are completely vulnerable to smaller ships and limit your combat role to only one trick.
If you know you're fighting a Python in a Viper then DFs are a way to give you a decent fighting chance but if the Python is ready for you and competent then you're going to be in for a very tough battle and you'll probably run out of missiles before the shields go down.