Sadly, there is: IPv6 requires retraining your staff, and running dual-stack inceases the testing and administration workload.
IPv6 also brings the additional requirement for ISPs that they can't allocate a single outside address anymore, so no way to enforce one-machine-per-residence limitations (I wouldn't expect an ISP to try this in 2016, but it was still common five years ago).
> so no way to enforce one-machine-per-residence limitations...
Eh?
Have you ever seen an ISP that did this and wasn't foiled by attaching a NATting router to the ISP-provided bridge device? [0] (If the ISP is providing the router, then it's trivial to only allow traffic from either the first MAC address you see, or the MAC address configured by the customer during some setup procedure.)
Of course it's technically trivial. But that didn't stop PHB's from inserting such language in consumer contracts, so why should we assume that the same PHB's would happily embrace IPv6, since "with IPv6, we must provide every one of our customers with 2^80 addresses"?
IPv6 also brings the additional requirement for ISPs that they can't allocate a single outside address anymore, so no way to enforce one-machine-per-residence limitations (I wouldn't expect an ISP to try this in 2016, but it was still common five years ago).