Kate, FWIW, this type of product has been used many times on the TV shows. Of course we don't get to come back to those places 20 or 30 years later, but the patch material will probably outlast the house. I've got some in my house. This is in a non-structural area but I would not be concerned about using it there.
That said, then you get to usage issues. You have to get down to good wood to make a proper repair. Some material suppliers offer a thinned epoxy to soak into rotted wood as a partial replacement. Personally, I would rather just get rid of all the rot and then replace. There are 2 different basic kinds of epoxy and I do think that I would use the true epoxy with a chopped fiberglass filler for this application. It is several times stronger than the polyurethane type epoxy like is in Bondo and Minwax wood filler. Both kinds are stronger than wood.
You might want to brace the beam while you are working on it if you have to get to it from a ladder leaning on the beam.