I’ve been running the last available QTS beta package v.1.43.0.10389 from Dec on my TS-664 running QTS 5.2.8.3359 with no problems. I’m just missing the latest fixes\features from the most recent release that’s having new build issues that prevented a QNAP package and those are pretty minor.
I typically stick with public releases these days unless a beta has a specific fix that I want to test or provide feedback on.
If you aren’t already familiar, I recommend pulling updates directly from Plex rather than using the QNAP App Store setup. Plex WebUI will let you know when there’s an update (you can set Public or Beta channel notifications in the Plex Server settings).
Also, if you aren’t already, I recommend backing up the PlexData folder. Plex can do regular backups but they go into its own folder - you can’t point them elsewhere like you can with Emby or Jellyfin. So you’ll want to back that up somewhere else. I use QNAP HBS3 to sync the PlexData folder over to another location once a week. Check out this QNAP FAQ about getting that PlexData folder easily setup: Qnap faq.....read me first! - #5 by ChuckPa
QNAP FAQ has good info but isn’t always easy to see unless you navigate to it: Qnap faq.....read me first!
I also switched from Synology to QNAP and that helped with some of the differences.
@dane22 and others still keeps it fresh. 
