In the UTM I had a "Public DNS Servers" availability group, which was listed in "DNS Forwarders". I found this to be the best way to maintain reliable DNS forwarders in the UTM. Currently there is no way to create availability groups in the XG, let alone use them for DNS forwarders. I miss this feature.
On UTM9, I always use that group type for Active Directory domain controllers.
David, I am missing this feature too. On UTM you have also option to check once the primary hosts fails and use the second one.
Really really a nice feature.