There should be both. Unit tests written by devs are not testing that the system does what it is supposed to do. They're testing that the code does what the pr

There should be both. Unit tests written by devs are not testing that the system does what it is supposed to do. They're testing that the code does what the programmer intended it to do. They have to be written by the programmer because they're coupled with the code. There's a separate testing process needed (as you say) that tests if the system does the thing it's supposed to do. The programmer cannot do this, precisely because it's basically testing their understanding of the requirements. I have my co-founders Q&A everything before rolling out to production. It's a pain, but it's saved us a few headaches where I was just following the golden path and missed some obvious problems.