Plex themselves literally recommend it and show a warning icon if it’s not accessible outside your local network. I always thought it was insane. To make matters worse their Synology package doesn’t auto update itself. I would guess there are a ton of vulnerable boxes sitting out there.
If you’re exposing anything on Synology to the Internet, you’re already playing with fire. Their packages (like Container Manager) are usually very far behind on security patches.
I think the current version of Docker engine Synology uses was end-of-life 2.5 years ago.
What get me the most, are the rabid Plex fanbois swearing that having jellyfin open on the internet is such a baaaad idea and you should spend ~50~ ~100~ 500 bucks for a sub to Plex which totally don't expose the same risk