I agree it works really well as a one shot, it is my goto for convention games. Saying that I've played Horror on the Orient Express (1st edition), and Tatters of the King which were both much longer games and it worked out fine. I think the longer campaigns support a slow descent into madness...
Call of Cthulhu is pretty bleak, you character is likely to die or go insane, and the ancient Gods can and do wipe people or entire cities out when the players can't delay them.