I think Ridley works best when he has a simple but solid story and an interesting backdrop (in terms of visuals) with which to tell it.
Where things tend to get wobbly is when the script is more mutable and not fully developed prior to filming. As any artist worth their salt will tell you, it can be very difficult to resist the urge to keep tweaking the details on a project you're not quite happy with. At a certain point, you have to learn to just leave it as-is or else you'll just make a pig's ear of the whole thing <insert obligatory George Lucas slight here>.
So yeah, the Martian is by no means in the running for best book for all time, nor the best Sci-fi or even Mars related story ever. But it's smart, well told, well paced, has a good sense of humor and a unique identity.
In other words the perfect foundation for a master film maker like Scott to really make something out of.