Ventile is wholly different than polycotton - treated or otherwise. The difference with Ventile is that it actually changes its nature when wet. The fibres swell and knit together greatly increasing its water resistance. It goes stiff as the fibres swell. So when dry it has large pores and can be comfortably worn. If you are "caught in the rain" it swells and, whilst not 100% waterproof shrugs off most of the wet. A wool layer underneath keeps you toasty and warm, even in a downpour. The thing is, you do then need to be able to dry it out, so if you are on some long trip, in permanently wet, cold miserable and unpleasant weather (you know - Scotland in the Summer) with no ability at night to dry it out, then its not for you. Snow - excellent. Alternating wet and dry - superb. As a jacket for a working man outdoors (farmers etc.), absolutely top. It washes and repairs easily, you don't get the sweaty fug of synthetics.
Lets put it this way - my £400 Harkila suit rarely gets worn. My Ventile goes on most days - even working outdoors in Winter rain, I pefer the Ventile with a wool shirt under. I may get a little damp leaking through - but less than I would sweat in the top end synthetics.