Interesting thread. I love in-world epigraphs just, like, a whole bunch. Give me all that worldbuilding! I'd even read a book with epigraphs that contain footnotes, and consider that a pleasurable experience. I suspect this is an extreme attitude.

I recall with Dune specifically, the reason why it worked was because Herbert was capable of a fair degree of insight, it was not so much the depth of the world but the fact that he could connect it to our own experience. At least to me, YMMV.