I’m trying using the Windows Store app on my TV instead of PMP, and the interface is surprisingly good – except for the UI elements that I can’t actually activate.
The Plex app seems to only accept mouse/touch input for some UI elements. That’s an accessibility problem – you can use the keyboard to navigate to different libraries, but many of them (for example, opening Settings, switching users, etc.) don’t respond to pressing Enter or the space bar. That’s also a notable problem when using a remote, since that surfaces to the device as key presses.
It also fails to respond to some of the media keys – you can pause and then press Back, for example, but pressing Stop does nothing.