Formal verification is always more valuable than mere testing, but it's hitherto more expensive. The thing that ultimately matters is closing the loop: how well do the tests match the requirements, both as written and as unwritten in the mind of the customer?

Working with an LLM has given me a real eye opener on unwritten requirements. It's like outsourcing. "Yes, you've given me what I wrote down, but I never expected you do to it in that way"