I can't talk about IT, but an ending can be unsatisfactory in so many ways. Whether it is a more classic trope, f.e. " it was only a Dream" or turning a comedy into a tragedy, both without proper foreshadowing, or it is too abrupt, all that can throw a reader out of a book, they enjoyed until then.

An ending is very important. I can forgive it, if a book starts a bit slow or is a bit weaker in the middle. I can't forgive a bad ending. In the former case, I can power through to the good parts or just don't finish. No hard feelings, someone surely loves it. But a bad ending, when the rest was fine? That sticks with me and feels like a betrayal. I liked you, you book and then you let me down? Nope. Bye.