Zpacks use silicone-coated 1.1 ounce per square yard ripstop nylon, (silnylon for short) as pack cloth. As Benjamin mentioned, this is not your typical pack cloth. If you're familiar with ultralight tarps or ultra light stuffsacks (such as those made by Granite Gear or Sea to Summit), you may be familiar with this fabric. Silnylon will rip easier than your traditional oxford reinforced heavy pack cloth; that's a fact of life. However, if you want a 5 ounce backpack, you won't find a single one that's made out of a heavy pack cloth.
I have a Zpacks Zilch and can vouch for the craftsmanship. These packs are handmade in the USA and it shows. I also want to mention that this pack is extremely well designed and executed. I got the padded, winged hipbelt option and it is quite comfortable, even with weights approaching 17-18 pounds.
The Zpacks Blast uses a heavy duty version of Cuben fiber, which I'm not familiar with, but the owner apparently thru-hiked the PCT with one so I guess he knows how to build a durable Cuben fiber pack.