For what it's worth, I found on two of my systems that had issues with the upgrade, if I followed the upgrade/activation with a clean install of Windows 10 (giving up the option to return to the previous version of Windows) the installation worked great. I had different but equally show-stopping bugs on my Windows 7 workstation and my wife's Windows 7 laptop until I did that.
On my wife's laptop I had even tried a fresh install of Windows 7 followed immediately by the upgrade and hit the same bugs as before (no "Modern" apps would load including Settings, and Explorer.exe was constantly crashing). A clean install of 10 fixed those issues and now it runs great.