I have a mibox s and it runs plex fine, there is however a known bug with plex (no video playback) and if it happens to you all you need to do is disable the "new video player" option.
Yes, that seems to be the case. This or the shield? ? I'm starting to think the shield as the mibox just sounds a bit buggy.
Getting off topic a bit but I have both the Shield and Mi Box 3. Both must have new player disabled in Plex at the moment because of Plex bugs. But Mi Box is more buggy by far, though for the price still happy with it.