I use Rainment all the time, but I got lucky and I got two of them, both 310s, both with int/disc, just so happens one increases armor when using a solar class, the other one increases armor when using an arc class.
As always though, wear the items that get you the highest light level. If I got a 310 raid chest, I would ditch the Rainment and wear a 310 helmet instead.
The Rainment also seems to have a bug in the text notes also.....when you level it up, it says you can increase the amount of Heavy ammo or Special weapon drops. I took this to be like the Titan Ruin Wings for heavy ammo. But when you mouse over the flavor text on the item, it says you can carry more Heavy ammo or Special weapons (depending on which one you pick). I haven't tested it to see which is the right answer......can you carry more ammo, or does more ammo drop for you?
Also, if you have Tlaloc and Rainment, the best class imo is arc. Using feedback, and as long as you aren't using your super, you will constantly be full on your grenades and melee. It's nuts.