While the colorful and vividly grotesque descriptions of various nightmares, alien cities and otherworldly horrors are indeed powerful, it loses a bit of its punch when we see the narrator describe almost every non-caucasian in the exact same way.
There's this great buildup describing Cthulhu, the grotesque Eldritch Horror... that "flopped" down the city stairs in pursuit of the sailors. Flop. While Cthulhu is terrifying the word choice evokes laughter.
Also that the great Cthulhu was thwarted by a boat to the head.
Lovecraft describes Cthulhu emerging under a "gibbous sky", a phrase that makes no sense whatsoever, since "gibbous" is the moon phase between half and full.
Values Dissonance: The non-white cultists are described in terms that make them sound more like savage animals then human beings. Whether or not Lovecraft actually was a racist is a matter of some debate , but his depiction of the cultists isn't much different than, say, D.W. Griffith's perception of "The Negro."