Re that video, a couple of points stood out for me. Firstly that the bag was soaked inside as well as out, and secondly that in the morning he - and the bag - was completely dry. So clearly the bag surface material was not acting as a water barrier. In temps down to -20C, I've had my Wiggys bag completely dry inside, but a thick layer of frost on the outside in the morning. Presumably the couple of pints everyone sweats overnight had passed through the bag and frozen immediately on contact with the outside air. (the bag I was using was a wiggys superlight, with a stated temp rating of 0F = -18C. And despite - stupidly - sleeping on a luxurylite mesh cot (which meant an airgap underneath me which is definitely not recommended in subzero temps!) I was warm all night. Although if anyone was heading towards, or below, -20C temps I'd recommend the Ultima Thule.
Re down, I've had a number of down bags in the past (and still have one - a Valandri Lhotse 1100 - luvverly). And yes, they are light, very comfy and warm when clean. But the "sweat" thing means they need airing effectively daily, which is not always possible. Furthermore, the night sweats, miscellaneous body oils etc mean the down gradually gets dirty, which damages the loft and associated warmth. Which means washing the bag frequently to retain the loft. And you can't just toss it in the washing machine like you can the Wiggys. Or store it in a compression sac.
Weight is an interesting one. Clearly, down wins the weight/temp competition. But synthetic? Having had a variety of quality synthetic bags in the past, I just don't believe most of their temp claims - if you take 10C off most claims you'll be about right. From a "book" comparison, Wiggy bags tend to be a bit heavier than the opposition synthetic bags making similar temp claims. Difference is that Wiggy's temps seem pretty accurate. so you're not comparing like with like. The old adage "carry light, sleep cold" might be worth considering!
There is one type of sleeping bag that hasn't been covered yet - the Buffalo bag systems, using fibre-pile as a lining. I've had these in the past, and they are a real option to other synthetic bags. You'll need at least two bags together to face serious sub-zero temps, but that does give you flexibility in warmer weather as well.