I think that using Refurb on new projects would be a great use case, but even for existing/old/legacy projects, Refurb can still be a good option, it really just depends on what is already there.

I agree that going and updating your company's codebase would probably be a bad idea, considering that there are some kinks here and there. Refurb tries its best, but it is very early on in its development, so errors that are emitted should be taken as suggestions, not gospel.