What will happen to my iPod Touch (4th Gen, iOS 6.1.6) if I upgrade to iCloud Drive?
It appears that if you upgrade to iCloud Drive, you won't be able to sync any game data with your iOS 8 devices. As an upgrade to iCloud Drive will no longer allow your older iOS 6 iPod to view or edit Documents in iCloud (which I assume includes game data).
Source: this MacRumors forum