I have used freshwater feeders as a way to entice some new animals to eat. My ray became sort of "hooked" on live food when I first got her and for a while wouldn't eat anything dead. That was a problem, so I would feed my feeders food soaked in selcon/zoe, and when the feeders were totally stuffed and about to burst, would feed them to my ray. Not the best way to get vitamins into her body, but it was better than nothing. Luckily she started eating frozen food, and once that happened, I stopped feeding live.
I don't think all freshwater feeders damage the liver. I have heard that specifically about only goldfish. I still toss ghost shrimp and veyr small crayfish to my ray once in a while. She likes to "find" them in the sand. If there's a good sale, I'll toss in some peppermint shrimp for her, too.
Ideally, you'd want to get your animals off live food if at all possible. Otherwise, nutritional variety (which is essential) is going to be tough to come by. And you could end up giving your animal an unknown parasite carried internally by a freshwater feeder.