Any wood, treated or not will fair much better if allowed to dry on all sides. It's the constant moisture when in contact with the ground or covered with roofing felt that does the damage. That said, any wood will cup, check and warp when exposed to the sun. Linseed oil/turpentine mixture will help but must be applied once every year. Spar varnish is good also.
The problem with most of the applications we tend to use wood for is that the planks are in a flat position which allow water and the sun to really do their damage.
Many years ago, treated lumber became widely available. I saw it used extensively for roof top decks and other structures exposed to the sun. The misconception about pressure treated lumber is that it does not need protection from the elements. The roof top structures did not last very long and quickly dried to the point of literally falling apart.
By the same token I worked on projects where wood was used to build outside steps, decks, etc, many years before I got into the trades. All of these structures were painted and built from close grain, probably old growth, Douglas Fir.
