This is roughly my experience with LLMs. I've had a lot of friends that have had good experience vibe coding very small new apps. And occasionally I've had AI speed things up for me when adding a specific feature to our main app. But at roughly 2 million lines of code, and with 10 years of accumulated tribal knowledge, LLMs really seem to struggle with our current codebase.

The last task I tried to get an LLM to do was a fairly straightforward refactor of some of our C# web controllers - just adding a CancellationToken to the controller method signature whenever the underlying services could accept one. It struggled so badly with that task that I eventually gave up and just did it by hand.

The widely cited study that shows LLMs slow things down by 20% or so very much coheres with my experience, which is generally: fight with the LLM, give up, do it by hand.