• the sentence presents two conflicting ideas:
• Her argument was compelling (strong, convincing, well-presented).
• It was based on outdated data (a major flaw).
• When you want to show that something is true in spite of a major drawback, you use a contrast connector.
• Nevertheless means "in spite of that" or "however".
• Corrected Reading: "Her argument was compelling; nevertheless (in spite of that), it was based on outdated data."