Interesting that this question just came up. Today I'm testing one of Lawson's roll top cuben bags for leakage during prolonged total immersion. I put a cotton T-shirt in the bag, rolled the bag closed, and submerged it for an hour in a bucket of water. Results: with 3 rolls of the closure, the T-shirt was a bit damp. Not soaked, but damp. With 5 rolls, the T-shirt was totally dry. Pretty waterproof, if you do enough rolls.
OK, so they're waterproof, but what about food odors? When I first received my order from Lawson last spring, I had the same question as the OP. So I took the bag that I figured would get the least use, and I put an open can of cat food inside a quart sized freezer bag (to keep the cuben bag cleaner in the event that a critter totally had its way with the food). I tied it to a pole a good distance from the patio and let the locals have at it. After 3 nights, it was evident that critters of various sizes and inclinations had messed with my bag. It had been moved all around, flipped over, and it had 3-4 2 mm size bite holes in it. However, security of the bag itself, and the food inside, was never breached. So, it looks like cuben bags (roll top type) might be water proof, but they are NOT odor proof. But they're darned tough and hard for a rodent to chew through.