I think Lord of the Rings might be a good analogy. LotR is sort of "hard fantasy" in that Tolkien put a ludicrous amount of work into building an internally consistent world, as you can tell by The Silmarillion, but that book is not enjoyable to read (in my opinion). Part of the reason LotR is good is that he took out enough of the walls of text to make it fun to read. A good hard sci-fi author might have a Silmarillion-level of knowledge about their own book's setting, but be able to leave almost all of that out of the final product.
If you want another example of actual hard sci-fi, I would suggest Lem's Tales of Pirx the Pilot. To give an example, I remember it being pretty much the first sci fi book that not only pointed out that laser beams are invisible in vacuum, but actually made it a major plot point of one of the stories.