...when the autos quit along with the computers.
Depending on the failure, the same 'flags' show up on the HUD as are on the head's down stuff, and the same associated info is rendered unavailable.
However it's not just a head's down repeater (with the exception of the tapes & HSI), as the displayed is info is 'real' world, i.e. 10 degrees in any direction in the FOV subtends 10 degrees of real world viewing. This is why EFVS & SVS adapt so nicely to HGS.
The center piece of this FPV discussion is whether you can have a true, fully inertial FPV, without the requirement for air data inputs, or not.
You can fail both the ADR 'sections' of the ADIRU's on an NG and still have inertial FPV. The angular effect of any inertial drift is minimal and not even a factor on an ILS as far as the beams and steering command, although with inertial drift the FPV may not exactly visually overlay the instantaneous real world aim/impact point, but the steering command will be centered in the FPV on LOC & GS.
You can fail 2 ADR's on a 330 and still have an FPV, but not 3 ADR's, it's not a matter of being reliable or not, it is just flat taken away.
Notice the AOA in the upper right corner on the NG.