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.
Are you forced to use them? If not, where's the problem?
I for one love ipv6, it restores the end-to-end principle.