Apparently we do use it for different things. One example is Raymond Chen’s excellent blog The Old New Thing, which has formatting that often breaks completely on RSS, to the point of being unreadable (he does not have any influence on the blog software as far as I know, though). Generally, I often noticed that articles look (and sometimes convey their content) much better on their actual websites, since that’s what they were formatted for first most.
Much more infuriating is that the various “Reader Modes” in browsers and e.g. Pocket sometimes not only tends to omit images, but even whole paragraphs of the text. It’s hard to notice that you missed a paragraph that you don’t know existed, so I don’t trust them anymore. I believe that should be less of an issue in RSS, since it of course includes the text to be displayed in the feed. Still, since I cannot be sure that authors take full attention to their RSS feeds, I prefer reading them on the website just in case some other formatting quirk messes things up.
Stuff like info boxes or illustrations might not even be part of the main body.
In that way, I actually prefer it when the RSS feed just contains a meaningful preview. Though I guess the correct solution would be to include both a preview and the full article, designated as such in the structure.