I have weird issues with Neil Gaiman. Now before anyone says anything, I love, loved The Sandman series. Those comics pretty much introduced and cemented my love for Neil Gaiman. I also love Coraline and The Graveyard Book, but for some reason, I just can't get into his adult books. I've tried both American Gods and Good Omens because everyone was praising them to high heavens, but I just couldn't get into them. Don't know why; there wasn't anything specific I could point to where I said he lost me.
I have a similar thing with Kurt Vonnegut. I like his eight rules for writing and all the quotes I've seen attributed to him make him seem like the kind of guy I'd really like to read, but I cannot get into his fiction.
Ah, you've hit multiple points near my heart!
Gaiman: I'm the exact opposite of you. I loved American Gods and Anansi Boys, and Neverwhere not as much but still enjoyed it. I wish I had the guts to write like this and pull it off. His kids stuff? Not a fan. Coraline was cute but not awesome, and I couldn't get into Stardust at all.
Vonnegut: I truly believe Vonnegut's strength was in short stories and not novels, although I would still read every single novel he wrote if he wasn't "up in Heaven now." (If you read Vonnegut deeply you'll get the joke.) I also think he wasn't that good of a sci-fi writer. His best stuff was modern day contemporary such as Dead Eye Dick, Mother Night and Bluebeard. I don't remember any of the Billy Pilgrim unstuck in time stuff from Slaughterhouse 5, but remember the WWII stuff vividly. Interesting tidbit: Player Piano, his first book, wasn't written as sci-fi, but as satire of General Electric. The publisher had him make some changes and billed it as dystopian sci-fi because they thought nobody would believe that real companies act this way.