I've had so many ups and downs with Plex.
Running Plex on my Amazon Fire TV it wants to transcode EVERYTHING I want to play on it.
In turn, because I rip my BluRays without compressions, my i5 server struggles to keep up with the real time transcoding.
Yes I could re-encode everything but I really don't have the time or inclination.
For me, the fix was installing Kodi and PLEXBMC.
Now EVERYTHING plays perfectly, without transcoding.
Disadvantage is that the PLEXBMC interface isn't as responsive to changes (like watch status) and resuming playback isn't as accurate as Plex - i.e. it might resume 5, 10 of even 15 minutes before where you actually got up to.
Interface isn't a slick as Plex (like the delays in updates on watch status) and you'll need to make sure you choose a skin that lets you run in "PLEXBMC mode" (I think I'm just using the default skins which allows this).
Unless you have actual problems playing stuff via Plex there's no reason to switch. If you're having playback issues though PLEXBMC will likely mitigate, if not completely resolve, these.