Well, just to put my experience and 2-cents into the discussion: I used a cuben fiber tarp from zpacks with .54 strength and a bathtub floor with the 1.0 oz strenght over my entire PCT thruhike. There were stormy nights that almost made my tarp fly away, but it never ripped. The only time I actually damaged the tarp, was when I accidently punctured it with my trekking pole end.
So sure it is not indestructible but it is very tough when it comes to tearing. Of note is also, that the hole in the tarp never ripped more open, as the cuben stops the rips by its own design.
The real downside of cuben in my books is the cost over time, because it wears down way faster than sil nylon. A tarp may last for a while, but a cuben backpack is basically done after one thruhike, even with the best of all cares.