> For practical use you only care if it carries a certain amount of packets per second without excessive retries

The $40 black-boxes do two things:

    1. Basic electrical continuity test
    2. Read the marker chips (if present)

There is zero chance that a $40 black-box, even if it did a naïve packet pushing test, would deliver you reproducible results and introduces all sorts of other random variables.

"Excessive retries" sounds like exactly the sort of intermittent, hard-to-diagnose cable issues that you will not find unless you test the USB cable on a 6-figure rig. :)