Okay, let's back away from the details and try to look at the key disagreement.
Is your fundamental complaint not about mechanics but about the lore implications of the mechanics? If this Power was still 1 Refresh, but only showed up on high-Refresh beings of tremendous power, would it be acceptable to you?
It's about both, because the lore and the mechanics are not entirely separate things. For instance, the rebate for toughness powers isn't just a "lore thing." It's a fundamental aspect of the game mechanics that getting around something's defenses requires some amount of investigation, assessment, and leg-work to figure out and find something that works.
This stunt can strip the need for that away entirely. It's not just going against the lore, it's straight up removing a fundamental aspect of gameplay.
And, again, it's really, really lopsided when it comes to refresh cost -- for a -1 refresh power, the character can straight up negate defenses that otherwise might require 3-6 refresh to set up, plus devoting a number of additional resources like focus or enchanted item slots.
I'd feel straight up
cheated if I built a character to get Armor:3 or Armor:4 through magic, and the GM just said, "Nope, he's got a stunt that negates all armor." Also, going by how Armor spells are priced, you could argue that Armor:2 isn't a two-shift effect, but four.
Sure, but ignoring Armor:1 means 1 more stress per hit, whereas Claws or a +2 stress stunt means 2 more stress per hit.
Claws also comes with the additional caveat that you can't hide them without another power. Would this stunt come with a drawback at all?
Ignoring all Armor is only better than +2 stress per hit if you fight people with Armor:3+ more often than you fight people with Armor:1 or no Armor.
The issue is that Wizard or Crafter with Superb Lore who spent a ton of Item Slots to get an Armor:5, 5/session Enchanted Item, or even somebody like Eb who might have Armor:6 item (assuming he has Fantastic Lore).
Powers and stunts are not designed in a vacuum. And, again, ignoring armor entirely removes an important aspect of the game -- learning and exploiting weaknesses.
A standard RCV from OW page 88 with Armor 1...
Compare this to the Lethal Weapon stunt and Claws.
If you hit with a margin of 0, this Power still does no stress, Lethal Weapon still does no stress (+1, -1 for Armor, so total 0), and Claws does 1 stress (+2, -1 for Armor).
If you hit with a margin of 3, this Power does 3 stress, Lethal Weapon does 3 stress (3 +1, -1 for Armor), and Claws does 4 stress (3 +2, -1 for Armor).
IE, this power is the same or weaker.
So does having the power preclude having those other stunts and powers, then? Is it only an alternative? Nothing that's been said seems to indicate that.
ACAEBG does a lot more than this, and has a specific world role. And I don't know how optimal it actually is, cost-wise. Incite Emotion (Lasting, Potent) will be better than a Sword of the Cross against most foes, for the same Refresh cost (and no FP spent each time).
Incite Emotion is also limited to particular character types and, honestly, the general skeeviness of it has tended to keep it out of PC hands, from my experience. And ignoring even mundane armor -- which this stunt would include -- is one of ACAEBG's unique abilities, and potentially one of its strongest since the Raw doesn't include any other armor piercing except The Catch, and that's only on supernatural toughness powers. Aside from the Swords, a guy with heavy mundane armor doesn't have a RAW weakness (barring aspect invokes and such ... which, again, cost a fate point).
Yes. But assuming there is one (I can see several), what is it worth mechanically?
I would base it on the nearest equivalent, and balance it based on the possibilities -- maybe even scale it. Personally, I'd make it an item of power -- sword works, or maybe a hammer? (Warhammers traditionally were for getting around an armor's protection by just turning whatever's inside the armor to mush from the impact).
Let's go with the Surtur example (in part because I loves me some Norse Myth). In the myth, Surtur had a flaming sword. In fact that's sort of his only defining quality. So this Item of Power is a big, flaming sword, with a potential rebate of +2. Lets price this particular power at -2, so the discount is +1. But, Surtur's not as powerful as Capital-G and the angel crew, so he doesn't get the full, "This is the Catch to everything" stuff. It'd be the catch to, say, Aesir and Vanir related things because Surtur's directly opposed to those -- but for everything else, it has a separate power. Instead of ignoring all armor, supernatural or otherwise, I'd have it destroy specifically physical armor with a dedicated kind of attack (call it a maneuver that you immediately tag for effect). That fits with the destruction thing more than just ignoring armor -- you get hit by this thing, and your flack jacket or plate mail falls apart. With the bonus that the rest of your party can
also ignore that armor because it's been cut to flaming pieces.
Now, for other things? Well, it's a big, flaming sword so I don't have a problem with an additional stunt, maybe an additional -1, that gives it a +2 "destructive flames of Surtur" damage bonus in particular situations (maybe you spend a fate point for the scene, maybe against certain personal enemies, maybe it costs physical or mental stress, etc.)
I like something like that more because it being a particular item specifically limits it to Weapons attacks, and when you can feasibly carry around a bigass sword, it's more limited in where it's applicable, and it requires some kind of active invocation and cost to use rather than just an always-on effect. It's also more thematically appropriate to destroy physical things than get around metaphysical things for a being of destruction (I've statted up NPCs that had a toughness power flavored as, 'This guy is mostly intangible, so it's hard to do real damage to him,' where something like armor piercing makes less sense -- he's already smoke, penetrating him /more/ doesn't really do more damage ... although that guy's catch was fire, so I guess this sword would work on him anyway? Oops.).
And it still comes to -1 refresh as this is statted up.
So, in summary, rather than just a blanket, "Ignore all armor" effect, I feel it's better and more interesting to wrap it around the lore and theme it's based on, and limit it to affects appropriate to that.
As said, everything needs a story reason to work; rather than just, "what does it cost in a vacuum to ignore armor," design the power/item around its in-story will and purpose.