The problem with really long articles is that they presumably cover so much content, it’s always going to be hard to highlight the bits that we want them to be found in search for. Even more than with ‘normal’ length pages, it’s important to have the key search terms in the page title, the headline, the image captions, etc. This comes back to something which is at the heart of search engine optimisation nowadays: target a page at Google just as you’d target it at a real site visitor.
If you’re not going to get to the point, or reveal the topic, until the 10,000th word, you’re not going to get the attention of real visitors – and similarly, you’re not going to get the required comprehension from search engines. Real visitors want to know the subject of the piece at a glance, just as Google does.
If the article is thousands of words long because it covers very different subjects, you’d be much better off subdividing it into separate pieces, to target different searches. But that makes life easier for real visitors too. See how this works?