I've thrown a 20 year old .net 4.8 codebase that's a mixture of proprietary sdks and legacy code and LLMs choke. Building some greenfield pieces for the same application works great so far though.
I've also made some plugins for some hobby software with very limited viewing of source code that I'm very happy with.
we have a 6 year old codebase with pretty much everything proprietary and there's no AI that can keep up without having an entire team to create an in-house RAG and throw thousands per day in tokens and spend expensive developer time on reviewing and asking for stupid changes that don't fit in
these arguments would make much more sense if tokens were practically free and LLMs could run in pc graphics cards, but when the model requires a data center that can be seen in space just to give you 100 tokens per second then you're basically a car in 1900 when all the roads were dirt roads for horses