By what factor is the difficulty of the thin side pocket bank increased because it requires a much thinner cut than the straighter cross-corner bank?
For a player comfortable with thin hits, and the cb only 15in or so from the ob, I would say it doesn't matter. If you're guessing, or if the cb is far from the ob (as you described in your spot shot example), then absolutely thin shots are more difficult/iffy than thicker shots. Otherwise, with the shot pertaining to this thread, what's more important is the margin of error on the pocket end of the shot. It determines how accurately you must hit the ob, whether it's straight in or a 60° cut.
I mean, the straighter cross corner bank would have to be hit absolutely perfect in order to send the ob to a pocket that has than a 1° window of acceptance.
The thin back cut bank to the side pocket can be hit a little less accurately, as shown in the video, and it can still hit the pocket. As long as you don't whiff the ball completely, or barely graze it, it'll come on over to that side pocket every time. You can be off a little and it still goes. Whereas being off a little on the cross corner bank is a no go for sure.