You asked about relative stability between Win10 and Win8. I use BOTH so I can comment.
Win10 is vastly LESS stable than Win8.1 at the present time.
Why?
Well, what constitutes "stability" -- I would throw out three items.
First -- frequency and SCALE of OS updates. Win8,1 gets a few KBs from time to time, mostly security stuff -- that makes it "stable". Win10 has already gone through two major UPGRADES since it came out about a year ago, and from the postings on our sister Win10 forums, and on OEM forums that I frequent, this recent Anniversary "Update" has caused quite a few system problems, including crashes and outright failures! I can not even remember the last time that my Win8.1 system crashed or failed. IMHO, that makes Win10 a LOT less stable than Win8.1.
Second -- frequency and scale of new feature additions, and feature removals. My Win8.1 is essentially the same (especially since I have gone out of the way to BLOCK all the KBs from Microsoft that attempt to introduce spying into my environment) as it was a year ago. I haven't noticed the addition or removal of any features. Win10, in contrast, not only has new features in every major Update, MS has also seen fit to REMOVE features in their Updates, as well. This makes the feature base unstable in Win10.
Third -- behind-the-scenes changes FORCED on users of an OS version. I can not remember when the last time was that any of my settings changed in Win8.1. Once I got them as I wanted, they have stayed the same. In contrast, with the recent Anniversary Update, many folks have commented about how Microsoft actually CHANGED their settings -- without their permission! When the OS vendor goes behind my back to CHANGE my settings, without asking me first, that makes the environment UNSTABLE.
And OK, ALL the Win10 "fans" are going to whine on about how Win10 is in a constant "beta state" -- as an excuse for it constantly changing out from underneath us. But, if we concede to that, it is yet ANOTHER reason for saying the Win10 is less stable than Win8.1.