I know I've seen at least data channel working without STUN, this was the demo I think: https://github.com/cjb/serverless-webrtc/

If you say it's different with media channels, I'll believe you.

WebRTC can tell you your ip address without any external server, that only breaks if you are dealing with NAT.

edit: to clarify the NAT-less use case, i'm thinking of apps that can rely on/require p2p supporting IPv6 connectivity.