It is really fun to pursuit an illegal truck driver, especially with a ground vehicle in my opinion. However, I am not quite sure if the hydra users are the main issue for which the illegal trucker job is not that famous/played. If that really is the trouble... perhaps, increasing slightly the durability points of the truck itself will be a much better and practical solution than making Hydra's rockets powerless against trucks...?
As far as it is concerned about the drug trafficking job (the sea one), I see no reason for nerfing the hydra's missiles. It is not like every second cop owns a Hydra and not every cop is experienced enough to fly with one. On another hand, you pay a dozen buckets of money for the features of the aircraft that you have bought, if I am not wrong, the value of the Hydra is around 50 million. Therefore, it should be considered as an expensive vehicle, should not it?