They're toxic to garter snakes supposedly, I have no idea if this is true or not though.
A lot of frogs won't eat them because they will secrete a distasteful mucus, honestly I highly doubt ACF would mind them. I've not seen Xenopus turn down much if it's made out of meat..
I figure better safe than sorry and my frogs are big enough to down the larger nightcrawlers regardless. I figure with red wrigglers my frogs would either puke them back up because of the mucus or could be harmed if the rumor mill on the internet is true that they are somewhat toxic. That's just my take on it though, I could be easily be wrong about them.
Perhaps off topic here but I would really love to be able to add some more variety to my frogs diet. In the wild they seem to eat worms, small fish, terrestrial insects trapped on the water surface, tadpoles, recently morphed frogs, and even the occasional vertebrate however the vast majority of their diet is apparently arthropods and zoo-plankton, the closest thing that you can really find to this is krill, but I hear krill is loaded with thiaminase, as are shrimp, goldfish, ect so I steer clear of them.
Source (good read for ACF keepers, the diet of feral Xenopus Laevis in South Wales)