Specifically, it can access screen content, but not window content. As in, passing in the desktop hwnd gets you a picture, passing in a different hwnd gets you a bunch of black. At least as of when I tried it a few months ago on Windows 10.
All of the windows visible on screen. For my use case, I was taking a screenshot of my app (for automated testing), so I ended up taking a full desktop screenshot and cropping it to the window.