If I remember well at least 2-3 years ago it was complicated to measure IPv6 usage on a high-end router, and that was tied to billing.
Every vendor (Cisco/Juniper/Alcatel/Huawei) had a different way to do it and since B2B billing depended on it IPv6 adoption was not as easy as expected.
Every vendor had a different way to measure how much octets went through an interface on IPv6 using SNMP.
And the standard MIB (management information base) for SNMP only gave you IPv4 traffic.
So, if you wanted to measure the traffic you interchange with a third party on IPv6 you had to be tied to a specific way of doing in (some had private or experimental MIBs for that, in other cases you had to move the data through a tunnel and measure traffic inside the tunnel minus overhead.).
Very easy to make mistakes specially if there is a problem with the traffic late at night and somebody forgets to put you in the loop.
Every vendor (Cisco/Juniper/Alcatel/Huawei) had a different way to do it and since B2B billing depended on it IPv6 adoption was not as easy as expected.