I had a giggle fest on a dumpy core2 machine laying around where I took windows 7 32 bit home, upgraded it to 10 and downgraded it back to 7
4 times
first upgrade lost my USB host stuff and I was stuck without a mouse, downgrade back to 7 and still without mouse
uninstalled host driver, reinstalled after reboot back to normal
second upgrade it could not find my prolific USB to serial bridge, which is not uncommon, but downgrade back to 7 where it was working driver did not roll back ... again and had to manually install it
third time no hardware issues, decided my installed copy of visual C# express was bad and brokeded it, downgrade back still broke
fourth time flawless, so it seems windows 10 is learning, one day taking over the earth in a skynet fashion
btw everytime I downgraded back to 7 the first thing to pop up was a upgrade to windows 10 nag message