They are bad at architecture, even when given directives and tons of examples.
They will 1) fall into established patterns, which is fine if that's what you want, but extremely bad if that's not what you want or worse, when you're in territory where are not eestablished patterns and 2) the hardest/worst thing is leave all sorts of dangling state, impl. in the wrong places etc.
No human ever wrote a module and 'voila' - it's a constant process of iteration and refactoring - observing the situation, identifying paths to reduce complexity or state, grouping behaviours etc. etc. The best AI doesn't do this very well at all. It can do a tone of planning but it tends to very much be 'one waterfall iteration' and it's suggested improvements after that are generally poor.
It just does not work, we have to direct the iterations.