what i hate most about fungible power is it creates artificial conflict
even fantasy stories still have conflicts from motivations familiar from real life. The queen curses Sleeping Beauty, because her vanity and jealousy drive her to remove anyone prettier. Sauron wants power, so he conquers and subjugates. Nothing fantastical about any of that.
But ask, why does the Court attack Coyote, provoking Loup? Why is Zimmy in danger? Purely because of a magical rule of the setting, that their power, in the sense of influence over their environment, corresponds to power in the sense of something fungible like electricity. The artist even draws it like a transmutation of Coyote to an electric arc.
And like, there's really nothing in the real world that works like this? You can see this by contrasting to predation. A predator gets energy from the animals they eat, but in terms of energy, a human body is interchangeable with gasoline, despite the vast difference in "power" in the sense of ability. A predator gets nutrients, and can even absorb dyes, like how flamingos become pink, or toxins, like how monarch butterflies are poisonous because they accumulate toxins from milkweed. Notice first of all that this has only brought us up to the molecular level, any properties due to higher levels of structure are not absorbed. And second of all, that it's no longer fungible. Flamingos can't eat pink shrimp and use that color power to become green.
So how could you ever get a conflict between characters in a story, based on some predatory absorption of power? You can't really get a deadly conflict that way. A dead human is just meat, which you can get more easily from a pig or cow. The closest thing you can get is something like enslavement, which, again, is not fungible. You get the abilities of specifically the person you enslaved. Although most stuff you make a slave do is stuff anyone can do--so on the one hand, human slaves are fungible, but on the other, that means any "power" differential between characters is no longer relevant. The Hungry Choir in Pale works sort of like this. It gains "power" in the form of people signing themselves over to be waifs, and all waifs are basically the same regardless of who they came from. But then that power is fungible somehow, like Zed makes a box of Hungry Choir power. And Pale definitely does what I see as artificial conflict for power reasons, like why did character A kill character B, oh, to make number go up, even though they're not using character B's capabilities in any way, somehow them dying just contributes to this number like energy or money or something. Which is just a dumb reason to have conflict in a story imo, it's no longer a fantasy version of a real kind of social situation, there's a reason things don't work like that.