I use plex on both the new Nvidia shield pro with a 4k TV and older model Alexa cube with a 1080p projector. My brother has the server at his house, but we will be moving it to my house this weekend.
Alexa cube works great no problems
Shield pro:
A.I Upscaling of 1080p content to 4k is great most of the time. Sometimes spectacular. Sometimes just ok.
Until recently 4k content ripped from 4k blu rays, the colors did not display correctly. My brother reports a recent update to the plex client has fixed the problem
HDR Dolby vision processing is not working correctly. HDR10 works, and that is what I use for now.