After a significant number of iterations of testing, and improving it. When we wrote code by hand, something like changing an api endpoint or a simple front end change would still involve dozens of "write code" then "retest manually". This would not only get the work item done, but you would often encounter edge cases or other issues you hadn't thought of when writing/reading the ticket.

When AI can one shot the implementation and testing, it likely only gets 1 round of human testing if you are lucky. Then scale this to an entire AI generated project, the ratio of features to manually run tests is astronomical. In the old days this ratio was inverted, and the tool has been battle tested before reaching any users.

Nothing is foolproof, but signals can go a long way. If an app's UI is poorly considered or comes across as careless for example there's a good chance that its other aspects are like that too.