“99% success” is too vague to be especially meaningful in such different use cases.

99% successful web development where the 1% missing is security that and the company leaks its entire repository of user data every few months is not good.

99% successful car driving where 99% of 5-year periods have zero accidents per car is an order of magnitude safer than the average driver. (And gut feeling, I suspect approaching the tail-end of the safest drivers on the road.)