One of iOS' more annoying quirks is its opacity and lack of user affordances.

The volume buttons don't seem to affect haptic noises, but the mute button does. The OS locks up for about a minute when you change passcodes while it re-crypts everything, but from the user's perspective it just looks frozen. The implications of opting into their 'find my...' and iCloud networks are not adequately explained. The devices take 15-30 seconds to actually cut power after they appear to shut down, and during that time they won't respond to any button presses.

The list goes on, using a modern iPhone feels like you are constantly negotiating with a disingenuous UI. The OS regularly claims to be done with something while it continues working furiously in the background, and it is full of undocumented exceptions to common settings.