Modern anti aircraft warfare is a lot more complicated than one might think. First, you have to detect the aircraft coming. Not always easy, when you consider stealth technology and the fact that radar is prone to not work in bad weather. Second, you have to "acquire" it. Not a lot of people know for certain what this means - it means you identify, "yes, its an enemy aircraft. Yes, it's coming towards us." and you get a lock on it with your AA weapons. Third, you have to keep your track on it long enough for the guns and missiles to do the job. This is tricky because any pilot will be doing his damnedest to avoid the flak, using ECM, dropping flares to misguide heat-seeking missiles, and dropping "chaff" - strips of foil, to fog up the radar. Also, they aren't always easy to see unless it's a clear day - night and bad weather make it tricky to see the target, let alone shoot them down.
And just because you've hit it, doesn't mean its dead. Freeza is absolutely wrong on that point - even in WWII aircraft were neither frail nor easily destroyed, even more so today. Modern jet aircraft are overbuilt and armored to last. Weight isn't nearly as much of an issue with the advent of jet engines. Many aircraft have duplicate or even triplicate systems - you shoot out his targeting computer, and another one located elsewhere in the craft takes its place.
Little-known fact - when the Germans first came out with their .88 anti-aircraft guns, it was believed that they'd require about 50 shells to take down one aircraft. In practice, over 12,000 shells were expended for each bogey taken down. Missiles are more effective, but far more expensive.
It's far from C&C's "shoot missiles at it until it dies" strategy. In practice, it's very difficult to take down an aircraft, especially before it drops its payload.
All this is assuming we use aircraft in OP3, which I think isn't the best idea.
