I use Homestream which is Sony's repackage of Serviio and fast forward, rewind works fine DLNA streaming from PC to my HX850. From what I recall, Plex Media Server and PS3 Media Server were also able to fast forward and rewind very nicely.
It is possible that video is being transcoded by the DLNA server before streaming to your TV and if that overloads the CPU on your PC the DLNA server may not be able to respond to FF, REW commands from the TV. It is also possible that the software you are using for DLNA streaming does not support FF, REW but that is rather unlikely.