While China represents huge scale as a market, it has fended off the entry of just about every major Western technology start-up. Uber, a five-year-old company that operates in more than 310 cities and 58 countries worldwide, faces homegrown Chinese rivals like Didi Kuaidi, which has more than 90 percent of the market and is backed by two of the largest Chinese Internet companies, Alibaba and Tencent. It may also have to grapple with a fickle central government that could shut it out of the market overnight.