Human written code contains as many bugs as human written code
Seeing as these "AI programmers" just digest large amounts of human written code, it's a mathematical certainty that they will produce code that contains as many bugs on average as the code they have digested, as all that produced code is itself an average sample of buggy code.
If it's to be anything more than this, the AI code assistants need to be trained in the process of debugging, by learning to identify common classes of errors, learning how to fix them, and doing this to all code before feeding it into the main code producing engine. Then it might be Garbage In, Less Garbage Out. But debugging is often hard and non linear, and needs real intelligence to work out why things happen, so this is going to be incredibly difficult to simulate algorithmically, which is why I suspect they will try and minimise this downside when hyping up the code churning "co-programmers".