well, the airship only opens up at astronomy, so focusing them early game will leave your military lacking. Besides that, it's got about the same specs as the queen of the line, with less attack strength but with flying. It's just as fast, and certainly not unattackable (not more than druids/rangers/water walking cultists). Just throw some fireballs at them and they'll come crashing down (on mountain and sea tiles anyway, where the units can't leave the ships to defend (this should be removed anyway, loading and unloading shouldn't be possible at 0 movement!)).