This is why I always have my visualizations overlay both failure rates and usage.

A given failure rate (or even worse, a failure count) doesn't tell you much about the system without also including the totals of both success and failure.

I'm sure I could be more rigorous with this though. Is there a way to express a given failure rate in terms of certainty? As in, we have sampled the failure rate of a component with fixed parameters a, b, c, and we are x% certain of the failure rate? (Maybe I'm wording this wrong - I don't have much of a stats background).