In my experience, they're better at poking holes in code than writing it, whether that's green or brownfield.

I've tried to get it to make sections of changes for me, and it feels very productive, but when I time myself I find I spend probably more time correcting the LLM's work than if I'd just written it myself.

But if you ask it to judge a refactor, then you might actually get one or two good points. You just have to really be careful to double check its assertions if you're unfamiliar with anything, because it will lead you to some real boners if you just follow it blindly.