Complex sentence: The SAC senior leadership had been dubious about the type from the beginning, although its crews eventually became enthusiastic about the aircraft, and while its performance and design were exceptional and appreciated, it was never easy to fly.
Split into simpler sentences: The SAC senior leadership had been dubious about the aircraft type from the beginning, although its crews eventually became enthusiastic about the aircraft. While its performance and design were exceptional and appreciated, it was never easy to fly.