marine snails are most definitely not equal to FW snails, which are definitely a complete PITA. marine snails can be difficult to correctly ID, so be careful when buying them. one common mistake is to buy snails from the colder waters of the Pacific and drop them in your reef tank. They'll live for a while, but will eventually "cook" in the warm waters of your tank.
Coming from FW myself, I always hated snails, and it's been a slow transition for me to accept and even like having snails in my reef tank. but if chosen correctly, they are terrific clean up crews. Get a variety of species so you cover your bases as to what you want cleaned up (various algae, detritus, carrion) and you have made a great step forward in making your tank more self-sufficient and less labor intensive for you. and no, you shouldn't have a snail graveyard either. Sounds like something was wrong in your friend's tank for that to be happening. Maybe they had scattered empty snail shells around for hermit crabs to use?