Your right,earthworms are far better,the fat content in these super worms isnt the best thing for a fishes liver,i think they are good however in moderation as part of a healthy diet,not to be fed exclusively.
The are pretty fattening but good as part of the rays diet.
Superworms' acid is pretty corrosive and they will eat through anything even styrofoam so handle with care, as they spew acid when handled which can be pretty itchy.
The acid somehow aids my rays digestion too, but thats just a guess. They are also handy to feed vitamins/pellets to, to transfer nutients to the rays.