If a group has more than 1000 items with unique permissions, we won't be able to determine if there are any external shares present in that group. If your group falls into this category, you will see this message:
We stopped analyzing this group's external sharing since it has too many items with unique permissions.
To crawl groups with external shares, the app must crawl the items with unique permissions within the site of each group. When there are over 1000 items with unique permissions in a site, it generates a heavy load on our resources which impacts the experience of all our users. To mitigate this, we limit the crawl to a maximum of 1000 items with unique permissions per group.
Note: It is not best practice to control the permissions of a group/site using unique permissions. We recommend that you use group memberships instead. Limit the use of unique permissions for specific cases, such as sharing a specific file/folder within a site to a specific security group/user.