I've been using Plex for a good 3 or 4 years now, and have upgraded through a good half dozen or so version upgrades. What disappointed me the most is that their support for subtitles has gotten progressively worse over time. I did all the tricks to try and make subtitles work, whether embedded or not, but in the end I found it easier just to navigate to the folder on my NAS and open the .mkv in VLC, which recognises embedded subtitles no problem!