I think adding new features is exactly the sort of place where AI is terrible, at least after you do it for a while. I think it's going to have a tendency to regenerate the whole function(s), but it's not deterministic. Plus, as others have said, the code isn't clean. So you're going to get accretions of messy code, the actual implementation of which will change around each time it gets generated. Anything not clearly specified is apt to get changed, which will probably cause regressions. I had AI write some graphs in D3.js recently, and as I asked for different things, the colors would change, how (if) the font sizes were specified would change, all kinds of minor things. I didn't care, because I modified the output by hand, and it was short. But this is not the sort of behavior I want my code output to have.

I think after a while the accretions are going to get slow, and probably unmaintainable even for AI. And by that time, the code will be completely unreadable. It will probably make the code written by people who probably should not be developers that I have had to clean up look fairly straightforward in comparison.