To be fair, this is Plex's intended use case. It is not a bizarre or unsupported config to expose Plex to the internet. It is designed to be on the network so that many different invited users can all log in and access your media. This model doesn't work if they all need to be on a tailnet or similar. Maybe Plex is less secure than nginx serving a web page, but its just a matter of a trust threshold.

If it were just me using Plex then it would be different, but I can't realistically expect all my friends and family to get on a tailnet, and so I take the risk with an up-to-date Plex on the internet.