I'm convinced they're bad UI based on how I've seen my parents try to use them on touch devices. They try to slide them horizontally, not realizing you're just meant to tap them. And the slide motion ends up doing something to the page or menu they're part of. At best it's a no-op, at worst the slide causes a transition to a different screen and they get confused.

They're antiskeumorphic. The design actively sabotages user understanding.