The Hellfire Rocket are very, very slow moving in flight. This makes them very vulnerably to being destroyed before reaching its target by any Unit that can Fire on Air Units with the exception of the Flak Tankand the Flak Turret.
The Reaper Drone has the capability of Air to Air targeting therefore may also destroy incoming Hellfire Missiles.
Since there are so many defenses that can destroy the Hellfire Missile it usually requires many Hellfire Vehicles all shooting at or near the same target for a Missile to get through the defensive fire. Once a few missiles do make it through it the defense will start to break, As the the number of defensive guns goes down the number of missiles successfully making it to their target goes up.
Other good Units to defend against Hellfire are Gatling/Elite for their range and rate of fire and Snipers ( to attack the Hellfire vehicles themselves ) for their great range and ability not to get distracted by the missiles.
Commanders should always be aware of the vast Splash Damage ( both intended and unintended ) done by the Hellfire missiles.
The Hellfire, Hellstormand Hellfire Turretare all just different firing platforms for the same class of Hellfire Missiles.
Hellfire Missiles are not classified as an "Air Unit" but falls in it's own class.
One of the Hellfire Vehicles greatest weakness is its extremely slow rotation speed. This vastly hinders its mobility making getting to the target a slow process and running from trouble an non option.
Hellfire's are armored bombardment platforms, perfect for raining destruction upon the enemy with long range rocket strikes
— In-game description
Cutting the weight of the rocket and increasing the output of the propellant allows the Hellfire to load and deploy rockets.