Honestly, I've seen it done pretty badly more often than well. Shapeshifting demons get used as this cheap 'gotcha' moment—a character you trusted for three books suddenly reveals they were a demon all along, and it just feels like a betrayal of the reader, not in a good way. It undercuts all the character development you invested in.
When it lands, though, it’s because the twist isn't just about the physical reveal. Take 'The Black Iron Legacy' series—the demonic shifts there reflect ideological corruption or the fracturing of a mind. The 'who' is less shocking than the 'why now' and the horrifying implications for reality itself. The plot twists because the rules of the world bend, not just because a face changes.
Otherwise, you just get a supernatural whodunit with fangs, and that’s been tired for a while.