I’ve heard that usability researchers don’t give macOS great marks, even though Macs have always had a reputation for being easy to use.
That’s likely because underneath the user-friendly point-and-click interface—the dock, menu bar, traffic light window buttons, etc.—there’s a ton of power user features that aren’t easily discoverable by poking around the GUI. You have to learn them by reading the docs, spending time on user forums, etc.
I doubt one in 10,000 Mac users knows that macOS supports a collection of emacs keybindings. What’s wild is that it’s not just macOS. They even work on an iPhone if you’re using a Bluetooth keyboard. For example, control-t will transpose the letters on either side of the cursor.