They're actually good.

Though the size and risk of a project do materially matter. If chrome ships a bug, that'll cause an impact on billions of people potentially, where as the typical software bug will impact a few hundred or dozens of people?

The way the code quality of webkit built up was by introducing a review gate. Chrome inherited its review gate from webkit, because it had established a much better track record for code quality.

The way you get any project to be "actually good" is by adopting methods to ratchet quality. Early WebKit did not have the same strict rules it has today, and we suffered because of that. Introducing review + test for every change rule was introduced, and the frequency of regressions dropped dramatically.