"Over-reliance on unit testing"? I have a somewhat different read from the inquiry board report, which says it was a "systematic software design error." Yes, inadequate testing, and a belief the working code for Ariane 4 meant it was validated for Ariane 5 were certainly contributory, but I think it's unfair to single out an over-reliance on unit testing.

The board argues that there was a bias towards believing the software does not have an error. Thus, any out-of-range value is interpreted as a hardware error, which means the CPU should shut down.

There was a decision to not include Ariane 5 trajectory data in the SRI requirements and specification. Thus, while tests were rigorous at the equipment ("unit") level, and there were system tests, they didn't test that case. This is test design failure.

In addition, the board says "the review process was a contributory factor in the failure."

I can see how those can be aspects of "over-reliance on unit testing", but it doesn't explain, for example, how some of the variables from Ariane 4 were protected from overflow exceptions but others were not.