> it was also not safe with only humans

Even if the average error-rate was the same (which is hardly safe to assume), there are other reasons not to assume equivalence:

1. The shape and distribution of the errors may be very different in ways which make the risk/impact worse.

2. Our institutional/system tools for detecting and recovering from errors are not the same.

3. Human errors are often things other humans can anticipate or simulate, and are accustomed to doing so.

> friction

Which would be one more item:

4. An X% error rate at a volume limited by human action may be acceptable, while an X% error rate at a much higher volume could be exponentially more damaging.

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"A computer lets you make more mistakes faster than any other invention with the possible exceptions of handguns and Tequila." --Mitch Ratcliffe