After this big rally, there's no beating around the bush: The fintech stock is expensive, trading at more than 42 times forward earnings, with nearly a $31 billion market cap. Nu got its start by offering a credit card with no annual fees through a sleek digital banking experience. While it's hard to imagine Nu keeping up this growth forever, it still hasn't penetrated -- and therefore really monetized -- much of its client base with some of its most profitable products.