Just thought I'd throw my hat in given that I do most of my gaming on the Steam Link these days.
- Four Xbox360 wired controllers work fine, I even had a mix of 2xwired and 2xwireless working for gaming nights with friends.
- I've found the Dualshock4 controller much better than the Steam Controller for a multitude of games due to the fact that it has a right stick *and* a gyro.
- Steam controller is still good for playing strategy/sim games. The DS4 has a trackpad but it's placed too awkwardly to be of much use in my current experience.
- I found that powerline adapters can be hit and miss. Wireshark captures were horrifying. 2.4GHz wireless is awful latency wise too.
- 5GHz wireless works great for streaming as long as you don't have other strong WiFi signals nearby. I run Link - wired - router - wireless - PC and it's almost as good as wired (the small amount of lag can mess up your accuracy with twitch aim and firing, though). Make sure the Link is the only device on 5GHz whilst in use if you don't want annoying glitches.
- I had some issues getting 5.1 audio to work. Make sure you're connecting the Link into your amp as Steam Link uses 6 channel PCM which can't be passed through HDMI control protocol apparently. Also ensure your speaker setup is configured as 5.1 in windows and that you have the SPDIF passthrough device enabled in playback devices.