Turns paragraphs within blockquotes into multiple blockquotes
Mangles your input— instead of preserving your source, when it collapses linebreaks into a single paragraph in rich text view, it deletes them in source view also. That is evil and very un-drupal.
Fails to save changes made in source view if you don’t return to rich text view
Puts ugly paragraph tags in source and generally makes source unreadable; WYSIWYG Linebreaks doesn’t seem to be able to ameliorate all of this in Drupal 8.