63Chevyll makes an important point. The biggest danger with sitting glissades may be the temptation for idiots to do it wrong, rather than the inherent danger. Either way, I’m no fan of sitting glissades.
I had heard about a few of the accidents that happened in that area and made sure that my father in law was well aware of the risks. Most of the time, my glissade was very controlled - I believe the shakiness of the video makes it look like I was going faster than I actually was. I did self arrest twice in one area, doing so as soon as the feel/hardness of the snow changed and before I got out of control. I came to a stop several other times without doing anything. Every time my father in law stopped on his glissade, he did so without self-arresting.
I guess part of my thinking that I did not express in my original post was that we started our glissade under what some would consider ideal conditions - there was plenty of snow that had softened up after being in the sun for several hours, we had equipment needed to stop our glissade if needed and experience using that equipment. Even with those things going for us, there was the potential of getting in a bad situation.
