I turned on several of the WP Smush settings and it screwed up a lot of my media files and caused some images to not show up on some posts/pages as featured image and several images not to show up within the post or on my index pages. Some images don't show up in the media library preview (even though the thumbnail in the library shows).
Note: The hard files are accessible on the server via direct link.
Some thumbnails don't not show up in the media library for a few (even though the hard files are accessible on the server). Some images now are also are attached to posts that they are not attached to at all.
Again, the hard files are all still there on my server in the uploads folder).
Some it didn't effect at all.
Basically, it made a mess of my media library and some images (and their attached posts).
Re-uploading all my images is not an option as I have too many posts.