Hole punching, which has various forms, may or may not work.
This means if you're doing something realtime, you may need to stick a server(reachable endpoint) in between it, at the very least reducing performance.
I have never seen any situation where this is not already necessary other than UPnP which already almost never works reliably. A publicly-addressable relay is already practically non-negotiable for anything over the internet.
For one, monopolies disabling it by default on their equipment? I remember some years ago having to guess the admin password at a vacation house so I could enable UPnP. It's usually framed as a security vulnerability, even.
uPnP fails when multiple devices are fighting over the same port assignments. uPnP fails when people have it disabled, as has been recommended many times over the years.
