Tried to configure a OpenDNS filtering service on my Fritzbox modem with an ISP which offers IPv6. Don't even bother.
OpenDNS won't let you register a IPv6 subnet. The modem prefers IPv6 DNS servers (because there's often no CGNAT). So the Fritzbox selects the ISP's IPv6 DNS server over the IPv4 OpenDNS server, meaning no filtering occurs.
OpenDNS does allow access to the resolver via IPv6, but this does not work if you have an account with filtering preferences.
To be clear, this issue is with OpenDNS. Not Fritzbox. Not the IPv6 protocol.
#ipv6