It is rather unfortunate it's left out that due to the previous iterations system critical drivers being copied over, it's recommended to clean install... or something to that effect.
The issue isn't Windows 10, as Windows 10 will function perfect when upgraded from Windows 7 or 8, and you can demonstrate this by installing Windows 7 or 8 in a VM and then upgrading the VM to Windows 10, as VM's don't require drivers to function in the fashion most users are familiar with. The problem arises from drivers, as all of the previous Windows version's drivers are copied over to the new windows directory. To simplify this, when an upgrade is performed it's around 75% new OS, 25% old OS. If you bring up my profile and click on my previous posts, scroll back to some of the first posts I made after I registered, as many explain this.
Almost all who've had issues are under the impression it's because of Windows 10 and they often state they never had this problem on Windows X. Essentially, drivers are programs that allow hardware to communicate with the OS... in other words, they're critical to the user experience. Without drivers, you not only wouldn't be able to interact with the OS, you wouldn't be able to see the OS. Windows 7/8 system critical drivers [drivers for anything attached to the motherboard] are not compatible with Windows 10 and when Windows 10 is forced to run with them installed, instability and wonkiness follows.
For any user to take the stance they should remain on a prior OS iteration has either been massively misinformed, or has not taken the time to fact check what they've been told. Again, Windows 10 is not the problem... users' failures to educate themselves on the proper way to install Windows are. I pity anyone still running XP as I will guarantee if one is, and they don't have a corporate contract, they've been hacked and any personal information and photos have been siphoned. Windows XP is insecure and has been since April 2014. Windows updates don't just fix bugs and exploitable code... they point a big red arrow at how to exploit a PC running any Windows version that hasn't applied the patch yet.