Interesting concept, with body horror, ultra-violence, teen angst, and poor/absent character development, and kind of falls apart when the first act's POV characters are introduced to the wider world. You know, like The Hunger Games. Or Gantz.
And no, fantasy stories aren't cursed to have shitty endings. Naruto was actually decent. Fullmetal Alchemist was a masterpiece. But let's also recognize that SnK hooked everyone not for its amazing writing but for the body horror, ultra-violence, and teen angst. We *want* SnK to be some sort of masterpiece of fiction because its story took on epic themes: world wars, genocide, racism, the spiral of hatred, etc. The 2nd act never really did justice to those themes because, well, that wasn't the author's forte. He was great at the whole "humanity in a bird cage surrounded by human-shaped giant monster cannibals" thing, but that's not what the second act was about.