"Companies on the cutting edge of React development like Facebook, Netflix, and Airbnb all readily use functional components. It seems pretty clear the upsides outweigh the negatives."
When should you NOT use functional components? When you have no other option.
programmingwithmosh.com/react/react-functional-co…
I have read from REACTs official docs that functional components, when possible, are preferable as standard, can't find the exact page now, but you can see the site above explaining it very well.