The Import tab of the Health Dashboard will display you any failures and expected failures this site faced or will face when importing (for an explanation see below):
If any import attempt has failed, it will be summarized here. These mainly include Drupal Entity API failures. Check out the Entity Status page to see what entities failed exactly and optionally trigger their Import again.
When configuring a Flow there are several configuration options available for only selectively importing entities. These include the "Ignore Unpublished Content" settings from nodes and media elements as well as the "Restrict to menu" setting from menu items. If an import has been ignored due to such a configuration, it will be summarized here. Usually you can just ignore all of these- in case you're wondering why an entity has not been imported without any error message, this is a good starting point. Check out the Entity Status page to see what entities were ignored exactly and optionally trigger their Import again.