The big problem is the weight: as TDP writes "There is no question that this lens can be handheld and that OS aids in doing so – but such use will be avoided most of the time by all but the strongest photographers. " It weighs a full kilo more than the Canon 300mm f/2.8 II and 200g more than the 500mm f/4 II. I tried it out and found it far too heavy for hand holding for me. Also, the IQ is ruined by a 2xTC, unlike that of the 300mm prime.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye