There are a few security issues with it, but all of the worst known issues require a valid login token. So an attacker would already need to have valid login credentials before they could actually do anything bad. Things like being able to stream video without authentication (but it requires already having a list of the stored media on the server, which means you have been logged in before). Or being able to change other users’ settings (but it requires already being logged in to a valid user).

Basically, make sure you use good passwords, and actually trust any other users to do the same.

You really shouldn't expose anything directly to the internet. It is a security problem waiting to happen. (Assuming it hasn't already)

This is how giant botnets form.

Nothing. People fearmonger