I just recently got a new LG B7 because the prices have collapsed, and the Plex app works on it very nearly perfectly.
Anything under 4K is fine, absolutely no problems. couldn't be happier.
And 4K very nearly works perfectly too. HDR loads up fine, looks gorgeous, video plays smoothly as opposed to VLC 3.0 as an example... But then it stops for a moment to buffer. Sometimes I get lucky and got 10 minutes of uninterrupted watching, most of the time more like 30 seconds, if that. The fact that it's totally smooth when it's running and how only the high end 4K HDR stuff is suffering suggests to me that it's not an issue of grunt on the TV's part, but just interruptions or limitations in the datafeed.
Keeping an eye on my network traffic when streaming shows that I'm only getting between 4 and 9-10 MBytes/second, mostly in the 6-8 region. While this theoretically should be enough for a lot of the stuff I'm watching (between 40-50 Mbps, 5-6 MB/s), it's weird that it doesn't ever reach higher speeds. My PC (the server), my TV and my Xbox One are all connected to the same router via ethernet cables, and when moving stuff between my PC and the Xbox One I get 70-80 MBytes per second, so at least I know that my PC can transfer faster, and that the router is fast enough also.
So it definitely feels like there's a limitation somewhere in the system. Is the TV's wired network connection that slow? Is the Plex server or app not able to use the full bandwidth available? Or could it be that the TV doesn't use a decently sized buffer, and would that be a limitation imposed by the TV or an app setting? Honestly, I'd be happy to wait for a few minutes while the app downloads a gigabyte into the buffer if that meant watching the rest of the film without interruptions.
FWIW out of interest I also tried the unofficial XPlay app, and the performance there was more or less identical, so it kind of seems like either a TV, app buffer size or server limitation.
If I could just get an interruption free 4K streaming experience (like I said the video looks gorgeous and is completely smooth when it's working) the app would be utterly perfect for me. HDR on Windows is such a non-functional hassle that streaming is pretty much the only way to actually get a usable experience.
Aside from that the app is working perfectly thus far, so good job on that already! Very close to the finish line. =)