I just ordered a custom square one from Joe at ZPacks. It is my first cuben product so I don't have any comment on downsides other than the obvious price and availability.
The cottage industry guys that are building stuff as a small business are a pleasure to support though. All of them I've dealt with are great people, most of them are guys who have hiked a couple long trails so they know their customer much better than the average company. I have a tent from Henry Shires (TarpTent), a pack from Chris at ULA, a tarp now from Joe at ZPacks, a stove from Russ, Rand and Lee @ Trail Designs. In all of these situations these small cottage guys make a better product than the larger companies for my use.
But back to your original question, the Cuben looks to be at least as durable as silnylon for tarp use. Abrasion seems to be an issue for the thinner materials but a tarp doesn't see much unless pitched in a way that causes it. Like anything in ultra-lightweight backpacking you are making an obvious trade-off to save weight but the tear strength seems to be sufficient when used appropriately.