LLMs are language models. So much of this can be reduced to a simple heuristic: If you can't think clearly, LLMs will not help you. If you don't know what you're asking for or how to express it precisely, it really should not be a surprise that the output is garbage. Failures of LLMs are more often failures of our own brain to consider the problem clearly. That's harder to admit than just blaming the AI.

It's important to note though, from the perspective of the LLM's objective function, that this is not a failure at all! LLMs are designed to match patterns. You give a jumbled mess of incoherent ideas, it will faithfully reproduce a token stream of incoherent ideas. It's only when the model output is subjected to the real world that it fails.