Do you think showing the Underworld is a “jump the shark” moment of any fantasy show?
No. Fantasy operates with a lot of archetypes and tropes. The Hero for instance. Whether literal or metaphorical, the hero tackles the Underworld/concept of death and is victorious, thus signaling his status as the hero. It can be powerful and very well done–just look at Harry Potter with their metaphorical crossing the threshold to the “Underworld” in book 1 or the more literal Limbo-like status of King’s Crossing in Book 7. Like Lily said, it’s about the stakes. The stakes are extraordinarily high in HP (of Buffy or SPN or even their katabasis moments of fantasy literature, like ASOIAF or the more urban and deconstructed world of the Magicians). The problem isn’t the appearance of the Underworld and a lord of said place—that is a pretty natural situation for OUAT given their adoption of the hero myth. The problem is that the stakes in the Underworld haven’t been made big and bad yet–Hades just sort of blandly threatens them (while being charming, yes) and Cerberus is too easily killed–and at the end there’s a bridge to Heaven. It also doesn’t help that the world building is fairly unimpressive and leave a lot of questions open.
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). The stakes remains high because it is not fully known what happens after you die and not everyone can come back on the whims of the writers. The people who came back, did it because of very clear circumstances that allow the reader/viewer to understand why this character was able to come back and that other character wasn’t.
"All your questions are pointless"