Complex sentence: The MiG-15 arguably had sufficient power to dive at supersonic speeds, but the lack of an "all-flying" tail greatly diminished the pilot's ability to control the aircraft as it approached Mach 1.
Split into simpler sentences: The MiG-15 arguably had sufficient power to dive at supersonic speeds, but could not do so because it did not have an "all-flying" tail. As a result, the pilot's ability to control the aircraft deteriorated significantly as it approached Mach 1.