I think the notion of WP based sites doing well in search comes from the fact that it makes posting content very easy. So you could end up with more content, and better content (ie with images and image galleries/videos) compared to hand coding everything Also because the market share of WP sites is so high (around 20% of world's sites run WP) google is probably optimised to crawl and evaluate that structure well.

But WP suffers with a few faults out of the box depending on template, for example category pages/tag pages spit out the whole posts which causes duplicate content. Some SEO plugins alleviate that problem. Also another problem i've heard is of stale content, where an old post becomes more and more buried and many levels deep further from the homepage. This only applies if you are planning to use WP in its blog format rather then use the Pages functionality.