Most user stories can be split. It may be hard to find a good way to split some stories, but most can be split. These are known as compound stories—stories that are made up of multiple smaller stories.
There is another type of story: the complex story. Complex stories are ones that cannot be split. They are inherently large or complex and there are no subparts to be pulled into separate stories.
Even with a complex story, you don’t want to let the story linger open for three, four or more sprints. Doing so
- Makes velocity less predictable from sprint to sprint
- Increases the risk of a developer going astray from user expectations , and
- Allows the team to develop the bad habit of leaving work incomplete at the end of an iteration
Use Progress Points to Identify Accomplishments
Read about it in this article: https://www.mountaingoatsoftware.com/blog/how-to-work-with-complex-user-stories-that-cannot-be-split