My personal opinion is leveraging LLMs on a large code base requires skill. How you construct the prompt, and what you keep in context, which model you use, all have a large effect on the output. If you just put it into cursor and throw your hands up, you probably didnt do it right
I gave it a list of the changes I needed and pointed it to the area of the different files that needed updated. I also have comprehensive cursor project rules. If I needed to hand hold any more than that it would take considerably less time to just make the changes myself.