There are numerous Magento extensions out there on the market. And the feedback that is heard often (from merchants and from third-party developers) that they lack in often stupid ways. Adding code quality assurance tools (PHPCS, PHPStan, testing, other analysis tools) to the stack definitely helps but still is not the final solution: Somebody could write a unit test where true is asserted to be true and you have suddenly unit test coverage. Somebody could write an MFTF test that destroys the features of others.
Some interesting questions: Is manual validation (code reviews) required? Are we talking only about technical aspects or is there more to consider? And what is more important - the personal link with a developer or the beauty of the code? Can we learn from bad experiences in the past? If so, how can we gather those experiences from everyone and everywhere? Or should we simply enforce MFTF to everybody?
How to move this forward? The initiatives of Magento Marketplace, Magento Community Engineering, ExtDN and many others are slowly moving to the point of separating the chaff from the grain. And we need to make sure this happens carefully, through discussions, like this DevExchange at Imagine 2019.