They're likely referring to scenarios like this:
https://news.ycombinator.com/item?id=8278864 - "IPv6 privacy addresses crashed the MIT CSAIL network"
& the much more complicated host discovery / address assignment process on IPv6 segments.
Still, I think I'd rather deal with some implementation kinks than intentional packet mangling like NAT.
- own a pseudo-TLD like .com.me and are really just creating subdomains
- have a large account / custom pricing deals with a domain registrar such that despite a high total cost, the marginal per-domain cost is approximately $0
- only internally-routable domains e.g. /etc/hosts or company-internal DNS server
- only alternative DNS roots like Namecoin or tor's .onion domains
About the price of a domainname? (Note I did not use the word "cost". I create dommainnames all the time at the price of $0.)
Given the choice between a domainname and an IPv4 adddress, I would take the IPv4 address.
Also, given the choice between a single, routable IPv4 addresses and a block of IPv6 addresses, I would still choose the IPv4 address.
IPv4 is "simplicity" in comparison to the complexity of IPv6. IPv6 has features I do not need.
Whenever I am granted the choice, I always choose simplicity over complexity.
Most of the time the additonal complexity is not needed and can only cause problems in the long run.
This is only the opinion of one "consumer". Certainly the "market" may have another opinion.