they are fine and I have never had a problem using them. I have fed raw fish, prawn, squid, mussel and various other types of seafood.
the salt in the food won't be any problem as fish and shrimp only have a certain amount in them at any time. Their kidneys remove most of it.
prawn is actually a better food than bloodworms and has less associated problems.
if you are feeding a gold nugget plecostomus then they should only get a small amount of meat food once or twice a week. They are vegetarian and need lots of roughage. If they get too much meat food then it can rot while it is in their digestive tract and this can cause bloating and internal bacterial infections.
You would be better off feeding the fish on pumpkin, zucchini, and various other fruits and veges. They will also feed on plants and algae in the tank and should have access to some driftwood.