Nearly everything that works in Windows 7 works in Windows 10, if you poke enough holes in it. The biggest exception to that is really just drivers, because hardware manufacturers have no incentive to update them past a certain point. There was definitely software that had to be updated to the latest versions to work, but that's where we needed to get to anyways.
I work in an environment that includes custom-written apps from the Windows XP era, and usually the absolute worst we hit was "hey, this device doesn't work on Windows 10, we have to buy a newer one". Not a big deal when it's a $6 USB-to-serial adapter, but people will look at you like you're crazy when you say the network is incompatible with an HVAC system. (Solution in that case, just take it off the network, you're better off.)
But believe it or not, you can still sync Windows Mobile 2003 devices with Windows 10 PCs, Microsoft's backwards compatibility is still insanely solid in most cases.