Mac repeatedly getting stuck at progress bar
Could be due to kernel extensions you have installed, I had the same issue and posted a blog tutorial on troubleshooting and fixing something like that:

https://smyl.es/how-to-fix-mac-osx-stuckhanging-on-progress-bar-will-not-boot/

To troubleshoot, startup your Mac in Verbose mode (after you hear the chime):


Hold down one of these key combinations on your keyboard immediately after pressing the power button to turn on your Mac, or immediately after your Mac begins to restart:


Command (⌘)-V

Continue holding the keys until you see white text on the screen.


Start up your Mac in single-user mode or verbose mode

Mac OSX Recovery

Mac Startup Key Combinations

Starting in Verbose mode should give you enough output information to debug the problem even further.  See my blog post for more in depth information and how I resolved my specific issue.

Cached versions of blog post:

Google:
http://webcache.googleusercontent.com/search?q=cache:https://smyl.es/how-to-fix-mac-osx-stuckhanging-on-progress-bar-will-not-boot/

Archive.org (WayBackMachine):
http://web.archive.org/web/20161218201511/https://smyl.es/how-to-fix-mac-osx-stuckhanging-on-progress-bar-will-not-boot/