I did more playing last night and tried to pay closer attention (I did ditch the companion). Most of the time it did work, but there were enough times to watch them follow me for a partial duration of SS and then turn around, and once in a while a full duration of SS would go with them still chasing me.

SS did work on snakes. It seems to have more problems with bosses, but that could also be because I use SS more often around bosses.