> it still has to punch through your router's firewall

That's why most routers use a stateful firewall. Then nothing has to "punch through" it just has to be established from the local side.

> block all incoming IPv6 traffic unless there is outbound traffic first, and provide little to no support for custom IPv6 rules.

This is why STUN exists.

> my bet is that there are close to zero popular games that actually use true peer to peer networking.

For game state? You're probably right. For low latency voice chat? It's more common than you'd think.