The issue with this is the same thing that happens with all sorts of data breaches, regardless of whether it was caused by negligence. Of course there's no pro

The issue with this is the same thing that happens with all sorts of data breaches, regardless of whether it was caused by negligence. Of course there's no problem if you do it right. It's just that guaranteeing you did it right is never possible with 100% certainty if you wrote the code, evaluated the code, and published the code. More eyes means less mistakes. More hands makes less work. If you roll your own templating system, and you mess up, then some text on a page is displayed wrong. If you roll your own crypto to protect customer data and you mess up, whoops, that customer data is out there.