According to Android Police, Vodafone Australia issued a notice on their support page indicating that Huawei discovered an issue with Nougat affecting the Nexus 6P, and this is why the higher-end 2015 Nexus got its update halted. The carrier’s support page says the following:
Of course, this still doesn’t explain the absence of updates for the Nexus 6 and Nexus 9 LTE.
The following is pure speculation, but the only thing I can think of is that the latter two along with the Nexus 6P all feature displays of greater than 1080p resolution.
Nougat might have some bugs to iron out that uniquely affect such higher-res displays. Perhaps it has something to do with the native real-time DPI adjuster. EDIT: since the Nexus 9 WiFi version did receive its update, this speculation seems unlikely.
Whatever the issue is, at least Huawei is aware of it and is working on a fix. Waiting awhile longer for the update is surely better than running a buggy Android build that could slow your phone down and/or drain your battery faster. Hopefully, the issue is resolved soon.