Invoking NAT "security" as a reason against IPv6 is a surefire indicator the person invoking it has absolutely no idea what they're talking about and should not be allowed within typing distance of any network infrastructure
But it _is_ a reason, and it _is_ true.
All my services and networks have IPv6. And my first operational issues with IPv6 were in 2008, when my Asterisk SIP server started failing after ~12 hours.
Culprit? Privacy addresses kept accumulating until they overflowed the SIP UDP packet size because it listed all the combinations of supported codecs/endpoints.
Oh, btw, do try to answer this message: https://www.reddit.com/r/VOIP/comments/131ex1x/ipv6_sip_trun... - it's still relevant to this day.