I agree with the notion that missing milestones are intrinsically considered failures. Being late, or unable to produce results, is never something that is desired. However, my own experiences missing milestones have been learning experiences, and usually, lead to structural changes that make similar failures less likely in the future. I feel that in any field, working to prevent a missed milestone is priority number one. However, having a contingency plan and being able to learn from such failures is crucial for long-term success and should be the focus if a milestone is being missed.