Yes this has long been a problem with Terragen, shadows within shadowed areas don't render well. Nor dose detail in shadowed areas (there's a thread about it somewhere). Soft shadows in the sunlight node do help as can multiple sun light sources and high GI settings but obviously this is not a desirable solution; the render times will sky rocket!. If the entire terrain is in shadow the renderer dose seem to be able to compensate to a degree and "find" the lost detail.
I've done a few searches on Google images to look for "Photoreal Landscapes" and there is not that much. Franks Cloud is the third image followed by a bunch of game, and Vue renders and f course "photos of real landscapes".
As you say Franck so much of our lives are recorded in photographic form that we have come to see the photo as a "real" representation of reality.
Hmm this is getting more philosophical that I was expecting ...
There are an awful lot of photographic styles, BW, sepia, high grain, high contrast, monochrome, the current HDRI and "Instagram" fads and I need not say, many more. I haven't checked but I would imagine there are Photoshop lens pre-sets for many cameras so in theory pretty much any render "could" be made photo-real.
I know that's not the point of the contest but don't you think it brings up some interesting discussions about the nature of reality and our perception of that nature. Ultimately the definition of "photoreal" will have to be subjective. As we have already seen we can disagree about this but in the end we will all have our own ideas about what is and is not "photoreal".
Cheers
Richard
now back to those clouds ...