Both Panoptes and Xol were canonically extremely powerful and dangerous in the Destiny universe, but they were relegated to weak single-player bosses that could be defeated in under a minute just like the Black Heart in D1. All three should have been raids, or strikes at the very least. Panoptes literally traveled through time to weigh out all variables that would result in a future where the Vex extinguish all life and turn the sun into a machine. That deserves more than a crappy story boss fight.