In reply to Suncream:
It's important to differentiate the two orientations of a figure of 8 bend, you have the one shown in the video with the ends of the rope on the same side (as per EDK/ Overhand which has extensive evidence of being fine, including this video, when joining ropes) and the flemish bend which has the ends on opposite sides of the knot (a'la the double/ triple fisherman's bend).
I am sure I have seen stuff before saying the flemish bend is safer than the offset figure of 8 bend, however given the similarity in tieing and the fact that confusions like this do legitimately arise I use neither for abseiling due to the potential of getting them mixed up when tired etc.
They also mention about blocking knots in karabiners/ maillons for abseiling with a tag line which seems a bit dodgy to me, I tend to tie a bight on the knot side of the maillon (figure of 8, overhand, alpine butterfly etc), clip a krab into this and clip the krab to the live rope (going from me directly up to the anchor with no knots in it) and then pulling on the tag line (noting this increases the risk of a stuck rope in some situations unless you are able to walk back from the base of the abseil).