My little anecdote about the relative obscurity of certain Fantasy authors. . .
Circa 1992, I'm first getting interested in D&D. I'm loaned a pile of D&D manuals by a friend, and start voraciously reading them.
I'm also first getting into Fantasy, with Tolkien being my first step, the first "real" fantasy literature I read started at the Hobbit and went into LotR. I found Howard's Conan books on our bookstore shelf and read a few (recognizing the name mostly from the movies, and realizing they were based on these books). If not for the Conan movies, I would have doubtless overlooked the books. Lovecraft I knew only from a few obscure references, it was many years before I knew what the deal was with him.
My mother is a very prolific reader, a die-hard bibliophile (our house at the time had a garage filled with boxes of books she'd read).
In Legends and Lore, among the other religions and mythologies, it had the works of Lieber. I thought it seemeed interesting, and my mother was keeping abreast of what I was reading in these D&D books (my dad didn't want me reading them at all, but my mom was cool with it as long as she kept an eye on what was in them). This big chapter references the works of Fritz Lieber and the whole set of mythology and history of his world, and it was the first literary reference she'd never heard of.
My mom, avid reader and devout bibliophile, had never heard of Lieber. She read Tolkien back in the 60's, Lovecraft long ago, and some of Howard too. A few books by other authors, but she seemed pretty well read in Fantasy literature. When she'd never heard of these books that Legends and Lore was talking about, I dismissed those books as some obscure D&D tie-in I'd missed out on.
I can't honestly recall ever seeing Lieber's works on a bookstore shelf, and this discussion was really one of the very, very few times I've ever heard him come up after that early brush with Legends & Lore well more than a decade ago. Therefore, I'm sorry to say that from my point of view, he's highly obscure.
By the same token, I only ever hear Jack Vance come up in discussion of D&D magic, and since people tend to react like D&D magic is completely unlike fantasy literature, Vance's works aren't exactly standing up to the test of time.