@Ladislav -
The OS is constantly panning, and is vastly inferior to (not even in the same universe as) Canon 100-400LII Mode 3 IS in terms of usability. It's constantly moving, so you have to fight it to get focus. I also haven't figured out after thousands of pictures whether, on a tripod, I'm better off with OS on, or off.
Now, I just leave it on because the keeper rate seems about the same for stills, and I don't have to quickly switch it if I take the camera off the tripod for a handheld shot.
On the Canon 100-400 II, I leave it on Mode 3, which leaves IS off until the moment the shutter is going to open; I've found that this is gives me the highest keeper rate.
On the Sigma 150-600C, I also cannot get a "sharp" (read: usable) BIF photo at speeds below 1/500 handheld; keeper rate falls off a cliff when I go to 1/320. But my bigger problem for BIF is actually that at 6.3 my 80D just focus hunts until the bird is gone if it's a "fast" moving bird. It works for big birds like eagles flying towards me, with the sun behind me. It's also MUCH easier with a gimbal.
The Sport is far inferior, IMO. It's so heavy that it can only be used on a tripod, and the price premium makes it a no-brainer to buy a 100-400L II + 1.4x converter, unless your goal is to use the 150-600 + 1.4x converter.
Weather seal, dust is a good point on the subject of shooting wildlife. There are lots of spots where dust is a concern. I haven't had any problems with the Contemporary, though, and some of my favorite birding spots are quite dusty.
About the switches: Yeah, they move like crazy, including the Lock switch, which is almost pointless, because it's just so easy to turn it off. Two other relatively minor complaints about the Sigma 150-600C - the lock would be much better if you could use it at zoom levels other than 150, and the tripod foot on the collar would be much, much better if it had a safety pin or a second screw hole. I find that on tripod, it will eventually loosen.
My final gripe about the lens, especially compared to the Canon 100-400 II is the focus ring. It's just too stiff and too thin to easily manually focus.
But hey, the price is absurdly good, the range is amazing -- especially on APSC, and the IQ, when you get everything right, is just fantastic. So despite all of my bellyaching, I think this is an awesome lens and an awesome buy. Being spoiled by the 100-400 II, which is triple the price if you throw in a Canon 1.4x extender, doesn't diminish its incredible value.