Another thing to keep in mind is that the module will use the featured image attached to a post in the module. If a featured image is not detected, it uses the first image in the post which may or may not be optimal. You might also see a mix of related posts with and without images.
I just wanted to mention that there is an option to disable thumbnails for Jetpack Related Posts under Settings > Reading > Related Posts. If you uncheck the "Use a large and visually striking layout" option, then no thumbnails will be shown :)