The Dark Eldar use this form of medium-heavy protection armour with an integrated force field. I understand the armour and force field rules as usually presented, but the force field for this armour is unusual. It states:

Ghostplate has a field with a Protection Rating of 20, but overloads after shielding against the attack if its user rolls doubles on any roll to deflect an incoming attack. The Soul Reaver , page 115.

OK, that's a little weird. Say you roll a 22 (or a 33, or any double besides 11). It's above the PR so it would normally bypass the field, and attacks that bypass the shield normally can't overload it. OTOH, the way it's written, I'm wondering if a result of 33 would both stop the attack and then overload the field. If this is the case, it's basically the same as PR 30 and following the usual rules.

As it is, it looks like the overload section was written-in by someone that was still thinking about the force field rules from Dark Heresy's Inquisitor's Handbook. If it just said PR 20 and followed the normal field rules it would be somewhat weak (for a force field, since it overloads half of the time that it stops an attack), but of course, the advantage to following the usual rules would be that higher craftsmanship ghostplate armour would have a more reliable field, and the special rules listed for the item currently resist such a clean ruling.

Also, there is a mention that Eldar fields tend to be very reliable, but this doesn't appear to be the case for the fields that PCs get access to. Perhaps a houserule to show this might be to all Eldar force fields to have the equivalent of the Reliable trait (if the d00 roll would normally result in an overload based on the craftsmanship of the field, roll 1d10 and the field only overloads on a 10). This means that the actual chances of Eldar force fields overloading would be 2% for Poor, 1% for Common, 0.5% for Good, and 0.1% for Best. Optionally, adjust the die result for overload as desired (using a 9+ for would give a failure rate equivalent to what is seen on the Autarch's force field in the Koronus Besitary).

Needless to say, if using such a houserule, the field on Ghostplate Armour would be fine at PR 20. This would also not apply to non-standard fields like the (very impressive but unpredictable) Shadow Field.

I think its just an effort to not make the Eldar stuff OP, but it tends to end up making it look pretty technologically stunted, unlike the fluff.

I've also noticed that at least one NPC wears Ghostplate Armour along with a Shadow Field. This also appears to be possible for Archons in the tabletop. If doing so, I assume that the field produced by the Ghostplate Armour will only become active after the Shadow Field drops. Would the force field of the Ghostplate Armour have a chance of stopping the shot that causes the Shadow Field to fail, or would it only become active for the next shot? Note that this question is only relevant to the Shadow Field, since it's the only field that fails by failing to stop a shot rather than overloading as it stops an attack.

You can usually only be protected by a single field at a time. Even though the shadowfield failed, its the field you were using against that attack.

Now it collapses, and the other one can kick in [it does seem to be an "always on with the armor worn" deal] or be turned on.