I’ve mentioned before here, but when I found a reasonable number of bugs in critical library used throughout one of the FAANGs written by their most senior and brilliant engineers by running a fizzier on it for a few hours, my opinion of those languages changed.
Integrating Valgrind and Clang’s checker, and running AFL for many CPU years was critical in unearthing some really funky crashes, hangs, overflows. Without those tools it’s not clear that those bugs would have been fixed. In fact, one of them affected me a few months prior and AFL helped uncover the codepath that was failing.