When is Scrum the wrong choice for a team, and what would you use instead?

Short answer

Scrum's Sprint assumes work that can be planned a fortnight ahead, held stable for that fortnight, and shaped into a goal worth committing to.Where one of those fails - interrupt-driven support, single-item research, hard external gates - a flow-based approach fits better.

This is a Scrum framework judgement question. The Scrum framework helps when the team can work toward a Sprint Goal, inspect a potentially releasable Increment, and adapt based on feedback. The Scrum framework is a poor fit when work is purely interrupt-driven, when the team cannot own the outcome, or when the organisation wants predictability without empirical adaptation. A strong answer respects the Scrum framework enough to say when Kanban, incident flow, or a different operating model is more honest. The Scrum framework is valuable precisely because it has boundaries.