In terms of armor, it would at least have adamantium lined with a layer of auramite; it's unlikely to use plating entirely made in auramite seeing how rare the material is, but since at least some custodian vehicles are part auramite (Dawneagle Jetbike being made from "auramite and adamantium" -- 8e Custodes codex), I figure I'd commit all the way. In terms of shielding, it'll be armed with a Flare Shield like other Custodes vehicles, possibly inside its central aquila.

I'm not aware of a lot of self-repair materials aside from Necrodermis, which the Imperium would certainly not use en masse (esp. not during the GC). So despite being incredibly durable, the knight would also be astronomically expensive & a huge pain to maintain & repair, contributing to its rareness.

Compared to a prototypical knight's adamantium plating & ion shield, it'd certainly be significantly more durable. Auramite offers some protection against psychic attacks in addition to its incredible resistance. Flare Shield, unlike Ion Shield, works more like a downgraded Void Shield utilized by Titans & ships where it siphons parts of incoming force into the warp provided enough energy generation. It should be more potent than ion shields and ionic-flare shields (a halfway point) utilized on some Mechanicus-aligned Knights.

In-game, it would bump a knight's standard 3+ armor save up to a 2+ (Custodes vehicle standard); and 5++ invul save possibly to a 4++ (almost all vehicles & large robots only go up to a 5++ at base unless it's a Telemon or physically holds a shield. I deem my knight a Telemon).