I spent two days looking for a good related posts plugin, I regret to admit. This is the one I finally settled on. At the start of my search, I probably wouldn't have considered it. But I learned plenty over the two days of searching .... enough to clarify what to look for. So that others don't waste so much time as I did, let me summarize a few of the lessons I learned:
1. Many of the plugins that insert thumbnails beside each related post require you to reset permissions on at least one folder to 777. That is a serious security risk. Eye candy is not worth being hacked.
2. The popular "Yet Another Related Posts Plugin" (YARPP) puts a heavy load on your database, at least according to WP Engine, who have blacklisted it.
3. The approach of generating related posts based on full-text analysis of each post's content generally doesn't perform well. You get peculiar matches or dated content .... Do you really want news items from five years ago appearing as related posts?
4. Several slick solutions are offered by companies in the "native advertising" business. They will share revenue with you to be allowed to include with your related posts some links to other, paying, websites. Google threatens to seriously penalize websites that use such links without full disclosure. Also, there is going to be a shakeout in this business. At least one company's slick plugin appears to no longer be supported.
5. The approach of bloggers manually selecting related posts for each of their posts is falling out of favor. One excellent plugin in this category appears to no longer be supported.
I think the best way to identify related posts is to merely select recent posts in the same category. This solution is automated. The posts, obviously, will be related. There will be no surprises. And you don't have to worry about dated material being included in related posts.
That is what this plugin (Related Posts via Categories) allows you to do. It is simple. It works. See it in action at the bottom of posts on my site http://glynholton.com.