Yes, drilling down definitely uncovers a ton of details – and allows you to avoid expensive design decisions exactly as you suggest. I’ve had similar experiences, being able to avoid expensive solutions when alternatives were available but not immediately obvious.

The primary benefit I see is that the very act of writing out the discrete tasks needed to implement the story can lead to deeper questions, opportunities for collaboration or reuse, and can chase out hidden assumptions.

For example, for one project, we discovered during task breakdown that a user story would require an integration with another system in order to keep things in sync as per the story description. After talking with the customer, they were quite happy to do this part manually, dramatically reducing the time and effort needed.

Without task breakdowns, estimates can be pretty poor, and stories can tend to grow in scope and complexity.