The Redledge Thunderlight is a ripstop nylon taffeta with a "waterproof breathable" coating. So it's not a separate gortex membrane with multiple layers over and under it. So it's probably not as durable as 2-3 layer gtx jackets but it's not designed to be. It's basically an expendable jacket that you use and abuse. At 55 bucks, you can afford to buy several, or one every year. When worn under a smock, as opposed to a stand-alone outer layer, you basically provide the other layers yourself. Think of it as a (removable) liner to your smock.
As far as water proof goes, I've worn it around the Carolinas in rainy weather and it held up just fine. No leaks or weird shit, like seam-sealing drying up or whatever. Gets kinda hot when moving hard, but what doesn't. I think Redledge is one of the great un-discovered gems of our time. I've had four of them in about 10 year's time. So they last about two seasons of heavy use. Probably a lot more with casual use.