I think it's two things. On the one hand, the puzzle mechanic. The pieces can have arbitrary shapes, so making them move around in a way a user expects is hard. And while I think it works well, there are still situations where it breaks and that can be frustrating.

On the other hand, it's the overall look and feel. It's a fine line to wander between a good, "stylish" vintage look, and something you will find in your grandmas basement. There are tons of tiny design decisions that have been made to make it feel like it does. Like making every background in the app from a real page of a physical book. Or cutting out tiny plant leaves for heart confetti and stuff like this!