Yes, they do get stuck in the impeller and such parts and die, which fouls up the tank even more. In addition to this, the snails I'm talking about seem to be able to breathe air, since I see them venture up there and gasp at the surface. If you get a disposable filter, I believe there is just too much current for them to float around and grasp onto a lettuce leaf.
They also can get sucked right out of their shells in the filter intake, which also kills them.