It's not hidden. There are 6-7 exotic armor pieces with the same perk. While it is true that they increase the carrying capacity for rockets or MGs, and they carry a perk that lets heavy ammo drop more often, Ruin Wings accomplish the same thing in a different way in the code. The way heavy ammo works is it will give a set amount of ammo based on the maximum capacity. Hence why low impact MGs get more bullets than high impact MGs just to name an example. Increasing this max capacity will increase how much ammo you'll pick up. The difference is Ruin Wings cause this ammo pickup to increase on their own, so they stack with a larger capacity for great effect. Example: A rocket ammo pickup ranges from 1-2 a pack to 4-5 to 6-8 depending on how those 2 perks stack. Not to mention a capacity max perk won't increase sword ammo, as they are coded to only work with their Scabbard perks. While it is true that you will get similar heavy pack drops with Alchemist raiment, they won't be as effective on a pack per pack basis. That being said, it still is an effective perk for its merits, and the fact that a warlock can have similar effects on heavy ammo is nothing to scoff at.