The fundamental limitation of LLMs writing code is that reading and understanding code is harder and slower than writing it. With other engineers that I work with there is an established level of trust where I do not need to deep dive into every PR. With LLMs it is like I am constantly doing code reviews for someone with whom I have zero trust. This is fundamentally a slow process, especially if you need to maintain this code in the long term and it is part of your 'core business code' that you work on 90% of the time. It also comes with all the downsides of no longer being an expert in your own codebase.

Ultimately I am responsible for any code I check in even if it was written by an LLM, so I need to perform these lengthy reviews. As others have said, if it is code that doesn't need to be maintained, then reviewing the code can be a much faster process. This is why it is so popular for hobby projects since you don't need to maintain the code if you don't want to, and it doesn't matter if you introduce subtle but catastrophic bugs.

Ultimately the tech feels like a net neutral. When you want to just throw the code away after it is very fast and good enough. If you are responsible for maintaining it, its slower than writing it yourself.