Office is not stable, at least over the long term. The hated "ribbon" marked a sad departure from the "stable" and efficient Word UI, and Microsoft's clueless regressions across the Windows platform have compounded the problem. For example, the deletion of the menu bar from applications. WTF.
I will agree that the Ribbon and what you can customize or use in it/looks has changed drastically since 2007.
But the Ribbon “paradigm” will soon have existed longer than the non-Ribbon “stable and efficient” interface so it is more “stable” in one way.
Ribbon aside, there are still lots of regressions in Word. The handling of styles, for example, which used to be excellent. Now Microsoft has stuffed the style list with dozens of asinine canned styles (most of which consist of different-colored underlines) that you can't get rid of. So instead of being able to format your document efficiently by selecting styles, you must now wade through a giant list of shit, looking for your own styles every goddamned time you want to apply one.
This crap really drives me to use markdown and pandoc to crap out word files when I'm done. I also really hate how word deals with document formatting and images.