I've used Plex for more than a decade, and I've seen it drop in quality over the years as Plex the company has pivoted towards being a streaming service.

Personally, my main beef is that as a client app, the streaming service stuff has bled into the personal library UI in obnoxious ways. For example, the default "tab" is called "Recommended". This shows some kind of algorithm-based feed of stuff they think you want to watch. This tab is the default even for your own private library. Why would it suggest stuff from a library that I curate? It's very strange. In the browser, it remembers your last active tab to some extent, but the native apps (like on AppleTV) does not. When I open the app and go to a folder, I just see recommended stuff. Similarly, if you go to the screen for a TV show in your library, it will show a "Related shows" section. It's my library, I know what is there, I don't want these spammy algorithmic things.

Occasionally I run into other parts of the app, which invariably try to steer me towards using their streaming service. It's just a constant reminder of how it's no longer designed for my purposes.