Flat design looks nice, but provides less affordances.

In practice, flat designs have less detail, so convey less information. This leads to uncertainty/doubt/cognitive load where there was none before.

Compare flat to skeumorphic designs:

* What gestures can I perform on this UI element?

* Where is the button/what can I click on?

-or better yet-

* What state is the iOS keyboard's shift key in (activated, deactivated, capslocked)?

Apple isn't the only culprit, Android/Material design is similarly flat and less usable.