“The infrastructure has gone to a whole other level,” said CJ MacDonald, founder of Step, a debit card provider aimed at teenagers. Introduced in September, Step quickly reached one million customers, partly from endorsements from social media influencers like Charli D’Amelio.In December, Step raised $50 million in funding. The company was not looking for more money, Mr. MacDonald said. But investors started calling as soon as the app joined the top-downloaded finance app list shortly after it was released. The money came together in a matter of weeks, he said.Investors are even clamoring to buy into broken deals. Plaid, which had agreed to sell itself to Visa for $5.6 billion last year, saw the deal unravel in January after facing antitrust scrutiny. Now the fast-growing company is in talks with investors to raise funding at a valuation near $15 billion, said two people with knowledge of the company who spoke on the condition they not be identified because the discussions are confidential. The Information earlier reported Plaid’s funding talks.Sheel Mohnot, an investor at Better Tomorrow Ventures, said Plaid’s sale price to Visa was viewed as “so amazing” at the time. But now, with multiple fintech companies approaching $100 billion valuations, it looks low.Some caution that the excitement has gotten far ahead of reality.Robert Le, an analyst at PitchBook, pointed to the valuation of Affirm, which has a market capitalization of $20 billion, or roughly 40 times its annual revenue. That’s significantly higher than the value that investors typically assign to blue-chip financial services companies. American Express, for example, trades at just three times its annual revenue.“I think it’s a little irrational,” Mr. Le said. “Over the long haul, some of these companies will have to come down.”Some of the start-ups have already hit growing pains. Chime, a banking start-up, had a series of outages in 2019, leaving millions of customers with no access to their money for hours. Some Coinbase customers have said they were locked out of their accounts or experienced thefts of their money. And Robinhood faces nearly 50 lawsuits and multiple regulatory investigations after it halted trading for some stocks during a frenzy in “meme” stocks in January.