[kovidgoyal]

calibre does use a system call to eject devices, but it's not quite the same thing as what happens when you use the safely remove hardware function, mainly because the USB subsystem in windows is so poorly designed that trying to figure it out was not worth the effort for me.

As far as being safe to disconnect, you can actually pull the cable at any time as long as no device jobs are running in calibre (that's what I do, though it is best to wait a few seconds after the last device job completes, just to be safe). The eject button is there only because I got tired of getting requests for it from people that were under the impression it was necessary.