The subject matter is pretty problematic, too. I think the "guest" on the ship and the way they traveled would get blasted by audiences (and probably even publishers) today.Yeah, there's a definite drop-off in quality. I think it coincides with the death of his wife and subsequent alcoholism. Both things that would've understandably affected his writing deeply.
That said, the first four (?) collections are astonishingly good. Like "if you haven't read them, stop reading this thread and order them immediately" good.