I’d actually disagree with using the S905X boxes from my personal experience, not because of how CoreELEC runs, it runs very well… but if you have a significant amount of movies and tv shows in Plex then you will absolutely experience crashes quite often.
For example, if you select the movies header then it will load all movies into a grid / list, when you go back to the main menu it will crash. Sometimes when loading episode posters on TV shows, it’ll also crash. These crashes are mostly eliminated with a S912 box, however if you have a lot of movies it will still crash when going back from the grid / list view. Haven’t tried an S905X2 box yet but I’ll be receiving one in a few days. Obviously the software isn’t as stable for that box yet (nightlies).
I’m not sure if anyone else has experienced these issues with Plex for Kodi, but I’ve tried it on multiple boxes and multiple version of Kodi (Krypton and Leia, separate builds including LibreELEC and CoreELEC).
If you do experience that crash, you can modify the library.py file in Plex for Kodi and set it to enable chunked view, reduce the chunk size to around 50 and you’ll notice significant improvement in load times and it won’t crash when exiting list / grid view. However you’ll need a S912 or more powerful to avoid the other crashes. Again, from personal experience but I have looked into it a lot.