A few months ago I messaged Chris at ULA about the durability of his cuben packs and here is how he responded:
"As to the Cuben, [...] it's much different than the cuben packs of years past where people used the lightest cuben possible and it self destructed after 500 miles. We only started using Cuben when then came out with the new, poly faced fabric, a little over a year ago. It's incredibly tough, we can't rip it with our hands and I can't puncture using our standard puncture tests (it takes too much pressure, once it gets over 50 # I fear for my knuckles if it does puncture) We had several people use the Cuben packs for the PCT last season, several did the JMT and a bunch were used on Andrew Skurka's Alaska trips, and we've had great reviews on all of them. We have no axe to grind with the Cuben, if it works, great, if not we wouldn't use it, but all the reports so far are extremely positive. I think if you can afford it I'd do it, if not, you can't go wrong with our Robic fabric. I for sure will be carrying a Cuben pack from now on, but I do like to have the top of the line toys.// Chris"
His testimony was one of the reasons why I trusted the cuben enough to buy a backpack (ironically though I bought one from HMG).
Still I am now considering ditching the cuben pack for a normal ULA one since it seems it may last me longer.