Assault would never be the same again if you turned on FF.
Imagine maps like Mazon?, put 1 guy above Front door with Flak to stop jumpers and the would be DNF/DNF.
Defenders with Rockets/Flak/Miniguns with slow moviong attacker in water? and as for those attacker running along the path, well they would only be able to fire 1, maybe 2 at a time at the most!
If any amount of FF was implemented it could be so that rockert launchers lost an equal amount of health as a hammer jumper, but even then you might rule out some launches as the health loss from getting launched and the fall could crater?
Like Lupus says, keep it the way it is.
If it ain't broke don't fix it, anmd as the like Astheno/Guardia/lava2 - well don't pick them?