Complex sentence: A few of the most heavily produced aircraft in history are commonly dismissed as barely competent types that happened to be ordered in vast numbers simply because of circumstances, but the most-produced types listed below are regarded as examples of outstanding merit.
Split into simpler sentences: A few of the most heavily produced aircraft in history are commonly dismissed as barely competent types that happened to be ordered in vast numbers simply because of circumstances. But in the main, the most-produced types listed below are regarded as ones of outstanding merit.