NAT breaks certain things, and broken NAT breaks them further. The specific thing I can speak of is VoIP or more accurately SIP. SIP was designed for the public internet and so there are NAT workarounds, such as: if my interface has an RFC 1918 IP, I’ll use STUN along with certain SIP headers and tags to indicate I’m behind a NAT and need special treatment by the remote SIP agent. Using public IP space as NAT foils this logic entirely.