Overall, Lovecraft can start his stories with some really interesting hooks as seen in "The Statement of Randolf Carter," "The Whisperer in Darkness," "At the Mountains of Madness," "The Thing on the Doorstep," "The Shadow out of Time," and "The Haunter of the Dark." He's good at getting you interested.
His ideas can be genuinely horrifying several fronts as is the case with "The Rats in the Wall" where ritual cannibalism seems to be an inadequate description of the horrors but something within the plausibility or "The Call of Cthulu," "The Shadow over Innsmouth," and "The Colour Out of Space" which go to the realm of supernatural/cosmic horror.
The Cthulhu Mythos and the overall ideas behind the dreaded and forbidden Necronomicon puts humanity and our own arrogance in place. I guess my only issue is too many people seem to know about the Necronomicon. His ideas are genuinely horrifying and the beings that use and abuse humanity are still horrific nearly a century after he wrote many of these stories.
I guess the real downfall are two fronts: The language is archaic, even for the time he's writing, which can make it difficult. At other times, the writing is just poor but when Lovecraft is running on all cylinders, it dark and bleak. Secondly, his racism comes out. It isn't even that he hates non-whites, he hates everyone if they are not a certain kind of white or attempting to assimilate to that social construction of whiteness.
Of the 22 stories in this compilations, the best were:
"The Rats in the Wall"
"The Call of Cthulhu"
"The Case of Charles Dexter Ward"
"The Colour Out of Space"
"The Shadow Over Innsmouth"
"The Thing on the Doorstep"
"The Shadow Out of Time"
"At the Mountains of Madness" was interesting but entirely too plodding. "The Dunwich Horror" had an interesting idea about civilized society and how precarious it is. "The Haunter in the Dark" wasn't too bad as well.