I have a wireless USB keyboard/mouse combo (Microsoft Wireless Keyboard 1000) which is working perfectly on a 2013 MacBook Air running Yosemite, the keyboard & mouse work perfectly - but only when the USB receiver is plugged into the right-hand USB port.
When it's plugged into the left-hand USB port it doesn't respond to the mouse or keyboard at all. The left-hand USB port works for other devices such as a USB hard disk.
So, the left-hand USB port is working, and the keyboard/mouse is working (including on a separate MacBook), but they're not working together. This is a pain due to the configuration of my desk, otherwise I'd just live with it.