One of the most difficult things for many people to understand about agile methods is the people orientation of agile. Those who are interested in agile processes all agree that process is a second-order effect on project success. The first value of the agile manifesto is that Individuals and Interactions are more valuable than Process and Tools.
This also alters the way in which you look at the capability of an agile processes. As Bill says, agile processes aren't about making a team succeed. So evaluating an agile process in terms of numbers of successes or failures is the wrong measure. The true assessment is whether teams find a process helpful to their success. In the agile view it's teams that succeed or fail, not processes.