But the books/shows you mentioned never fully described what happens after the person dies. The afterlife stays a mystery and the depictions are either in the forms of dreams (Harry Potter) or a vague memory (Buffy)
Those are only a few examples and there are more with clearer depictions of the Underworld: like the actual Greek/Roman mythologies. I don’t think showing the Underworld and what happens after you die is a bad thing (which is the heart of the question).It helps flesh out the shows own cosmology which is vital to world building. And just because you don’t see the Underworld in something like Buffy/HP doesn’t mean the viewer/reader isn’t made to understand what it’s like. Buffy’s song in Once More with Feeling–“there was no pain. No fear no doubt. Until the pushed me out…of heaven.” We may not see the shiny fluffy clouds, we but understand the emotion it wroughts in Buffy.
Where I do agree with you is in that the depiction of the Underworld in OUAT is largely “at the whims” of the writers. It’s not solid enough, scary enough, or well rounded enough for the viewer to feel like it’s worth knowing about. So far the only that the apperance of the Underworld does–along with gods, now–is muddle the cosmology of the Onciverse.