They are really death on grasshoppers and locusts. Ladybugs for aphids. Pasture rotation and mowing to control parasites and weeds. Sometimes you can wipe out a vector. I ran pigs into a pasture and they ate snails for a couple of weeks.
Snails are a vector for liver flukes.
This is anecdotal, but it seemed to work well. I have also used the diatomaceous earth, but feel that the best parasite control is sanitation and healthy vaccinated stock. Crowded and unclean conditions cause an explosion of parasites. I never tried to exterminate parasites, just keep them under control. Far too often they are treated as a panacea, but end up disturbing the natural balance.