alt Hacker NewsFable (and even Opus if kept tightly under reigns) does generate high-quality code even for highly complex tasks.
It generally performs better if the tasks are broken done into small manageable pieces, and the person is actually reviewing and calling out problems, which usually requires the person to be a competent engineer in the problem domain to begin with.
But yes, I have personally used it to build what the OP calls a JIT. I would usually write that by hand and it would take me one week. The AI does it in an hour.
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Your second paragraph invalidates your first.
If I need to be a domain expert anyway, the value of the tool goes down by orders of magnitude. Same if I need to first break the task down into pieces and keep reviewing all the output. That sounds to me like >80% of the work I'd need to do anyway.
If I need to design and understand all of the code anyway, I might as well skip the whole process of repeatedly fixing the subpar-at-every-level LLM output and write it all myself.
Personally, I've found the greatest value in asking for simple tasks, like wiring up APIs, generating boilerplate, bug finding etc. Anything that requires effort to do but results in either very little or very simple output, so that I can easily verify its correctness.
But give the LLM anything remotely complex to generate and it cakes its pants.
You can't make assertions about "quality" of code that was generated under an hour while it would have taken a human 40 hours, unless you put substantial amount of work into reviewing it.
I used Fable on a Zephyr project with time sensitive code for LR-WPAN and it broke everything. Literally made the code worst to the point that the devices stopped connecting.