Heh, maybe I'm a bit biased, as at one point I used to work for a startup(not YC) that was only 3 guys in an apartment, but also spent thousands of dollars a month on hosting for a site that had only a hundred or visitors a day, because they were expecting that any day now, "they will come" and viral growth will make them huge.

Being YC-funded is a boost, but without marketing and efficiency, it's not enough. There have been numerous YC-funded startups that ran out of money.