Reconciling a High SUS Score With a Low Completion Rate

A usability study on a new onboarding flow returns these results across 8 participants: completion rate 50%, average time-on-task 4 minutes (well above the 90-second target), average 3.1 errors per session, and a SUS score of 79 from a post-task questionnaire.

1. Explain how a design can score well above the 68 SUS benchmark average while also having a 50% completion rate. What does this combination most likely mean?
2. Which of the two results — the SUS score or the completion rate — should carry more weight in a go/no-go launch decision, and why?
3. What additional data would you want before finalizing a recommendation, and why would think-aloud data specifically help here?