I have a large collection of all MKV's and never use the mobile functionality of it other than using my phone to browse then play things to my home theater on the Chromecast. You can even tell it to not transcode and play in what they call, "original quality". The UI and interaction with your phone to find and play media on your TV is the selling (well.. its free) point for me. It's easily the slickest way to manage and play a large library of media.Ahh, right. Mobile and embedded device viewing. That explains it.
I've never had any interest in watching my content on anything but my home theater setup, which makes things easier.
In fact, I prefer if my content is never transcoded, and kept as close in format as possible to the source.
I usually use MakeMKV to pull the unmolested video and audio tracks of my choice off of my BluRays and then store them as mkv's on my NAS for viewing. It's a little hefty at 20-40GB per 1080p title, but that's one of the reasons I have lots of storage.
Some of that mobile integration, while I don't take advantage of it myself, is also super slick. I could be watching something on my TV, click the Cast To button on the Plex app on my phone then its now playing on my phone right where it was on the TV. Once I get back to the couch, press it again and it picks right up on the TV. That's a dumb use case but it gives you an idea how well it works.
You really need to just set up the server portion and play with it yourself. Its an incredible piece of software (multiple pieces, really) that makes library management and playback on anything super seemless. Gone are the days of needed an HTPC and Windows in your home theater. I've replaced those with simple Chromecasts.