I have a Tyvek tarptent. In sustained rainstorms the bathtub floor becomes a bathtub. I bailed out to a rainproof option (yay for group camping) and torture tested it - Tyvek, once wet, soaks up pounds of water and seeps through like a crazy thing, gallons of water poured out of the floor and the empty tent weighed in the pounds.

Not a great backcountry choice any time but summer - it might withstand a Sierra Nevada afternoon shower, not actual rain.