I installed Ubuntu alongside macOS Sierra and when I restarted Ubuntu (not grub) boots automatically. When I hold down the option key Macintosh HD shows up, and when I click on it, the apple logo shows up but nothing happens (the progress bar doesn't fill up). I do not have rEFInd installed.
Note that on Mac hardware, Refind is definitely recommended. You can install it as a package in Ubuntu, and then use refind-install to automatically configure it, and this should make your boot situation a lot easier.