There's a difference between not intellectually understanding something and not being able to refactor something because if you start pulling on a thread, you are not sure what will unravel!
And often there just isn't time allocated in a budget to begin an unlimited game of bug testing whack-a-mole!
My confidence in LLMs is not that high and I use Claude a lot. The limitations are very apparent very quickly. They're great for simple refactors and doing some busy work, but if you're refactoring something you're too afraid to do by hand then I fear you've simply deferred responsibility to the LLM - assuming it will understand the code better than you do, which seems foolhardy.
A lot of repetitive slight variations on the same easy to describe change sounds pretty good to ask an LLM to do quickly.