Small correction, if the game works on Windows 8, then it should work on Windows 10.
A friend of mine has some issues with certain games (I think mainly Games For Windows Live games) that will not run on Windows 8, and he has to boot into 7.
I have Steam , And I have been testing some games , I have over 420 games & 100 tools & music . I am using a AMD 8 core , it doesn't like It , hope to work bugs out as WIN 10 is great . Steam is a where if WIN 10 , said take one file out , but GO NOT GERENTE it. I Have not pushed Steam , as the DEV teams will have to deal with 10 , I do know they are using a 32 bit emulator .
What may not work now will most likely work in the future. Windows 10 JUST released, so the folks at Microsoft (as well as Nvidia and each game development) are working at it to properly optimize stuff for Windows 10. You don't have to upgrade now, but I do recommend doing so in the future.
With my current experiences, everything works fine in Windows 10. In fact, in many cases, games run better and much more faster. Just be sure to have a fresh install of each game and don't use backups.