I've been watching this thread and had the same question but I guess it depends on personal usage and plans. For me I manually update with Plex's offical Synology package installer (rather than the trailing edge version maintained by Synology) and that does what I need.
Have a google using 'plex docker why' and you'll get a few hits.
Well, it's been discussed a lot already why some will prefer docker than syno packages for 3rd party applications.
Synology seems to be ditching the 3rd party apps one by one so in case it happens to all 3rd party apps then some have already migrated to docker for these apps.
I'm interested in Docker and would like to learn more, that's one reason. And the reason that keeps coming back aswell is that third party packages refuse to work whenever Synology releases big updates to DSM.
I've used Plex as a SPK package for along time and yes, it works well.
I am using PMS in a container since years and it works like a charm. Everything is tidy and clean. I have 100% control of where files are stored. If a PMS version is messed up, I can revert to a previous version within a minute.
The only problem I had so far was caused by the number of preconfigured inotify filewatches. Though, this is a kernel issue, which applies to all linux based OSses and would've happend with or without docker. The fix needed to be applied on the host direcly.