most clams are strict filter feeders and most tanks cannot provide sufficient food in the water to sustain them for long, so yes it is true for the most part.
Freshwater and saltwater clams both can be successufully raised in aquariums.
Freshwater clams are far easier to keep as they dont require special lighting.
Clams can actually help the substrate as well. They bury into it and as filter feeders, help control detritus on the bottom.
yes they can do well if there is sufficient food but in general they don't because its not that easy to find a balance of leaving enough food for the clams without affecting water quality too much, especially if there is fish in the tank.