Aircraft are always very difficult to balance. Currently, they're very good in small fights, where there's ~<15 players in a base from any faction. Any more than that, however, and you'll always have one or two people as burster MAXes or heavies with lock on launchers. As Jelle said, you hit a critical mass of AA lock-ons and you completely wipe any aircraft in the air off the map. Currently, that critical mass is 3 HA's (or 2 if you're TR with the striker). This means that in any bigger fight aircraft are almost completely useless because they have to retreat as soon as they get locked on since there's no way of knowing how many people are locking onto you on the ground. In big fights getting locked onto happens more or less as soon as someone with an AA launcher sees you, and there are usually a lot of people with them around, especially when they're TR.
Aircraft are very frustrating to play right now, since your tactics have to be swoop in, hope the enemy renders early enough for you to actually line one of them up, fire and get out of there, hoping that you can get behind something before the enemy gets lock-on missiles off. If they do, you're generally dead since there will almost always be more than enough to kill you and they now intelligently go around mountains. If you're not dead, repeat when flares have recharged. That time is usually long enough that the guy you just killed have respawned three or four times already, though, so you actually have very little impact on any given battle.
The Liberator in particular is in a bad spot right now since it cannot flee with nearly the same speed as an ESF. It's in a rather annoying catch 22 situation where it's supposed to be a ground-to-air bomber, but if you're flying high you don't actually render your targets and thus cannot damage them. If you're flying low, you're instead such a large and slow target that tanks and dumbfire launchers reliably hit you, and two or three of those will kill you. In short, flying a lib is an exercise in frustration and I wouldn't recommend it.
Sorry for the lengthy and slightly ranty post, but as a pilot that's been constantly nerfed since release and having seen AA get constant buffs since release (except last patch where bursters were toned down slightly at range, Skyguard got buffed though), it's kind of annoying.