In China's hottest tech sector, hundreds of "online to offline" (O2O) start-ups, which draw mobile users to local physical stores and services, have failed as skyrocketing valuations deter investors and put the brakes on fresh funding. "The whole O2O concept is getting too expensive," said Weiwen Han, the leader of Bain & Company's Private Equity practice in China. "The valuation is very, very high. There's no traditional way to look at the valuation... because they don't have revenue."