[kccqzy]

No. Here's a simple strategy: the two peers send each other a few packets simultaneously, then the firewall will open because by default almost all firewalls allow response traffic. IPv6 simplifies things because you know exactly what address to send to.

[ectospheno]

That is my point. You hole punch in that scenario even without NAT. It is no easier.

[the8472]

It's easier since you don't don't have to deal with symmetric nat, external IP address discovery and port mapping.