Not OP and certainly not in the anti-IPv6 camp.

I'm wondering though: could you build a case that under IPv4, a misconfigured NAT would only result in lost connectivity for nodes behind the router, while for IPv6 a misconfigured firewall and worse triggered through a vulnerability, would then result in protected nodes being exposed?

I know NAT-PMP (port mapping) vulnerabilities exist, allowing external actors to set port mappings to hosts behind NAT, but this seems a bit harder to exploit than a bypassed firewall.