95% of YC-backed startups fail. A few go on to have decent success, and maybe 2 or 3 YC ventures actually went on to billion-dollar valuations (Dropbox and Airbnb). And that's considered a really good success rate.
Since we're talking here about what you call 'unicorns' (I hate that word), then we're talking about the odds of building a billion-dollar company. YC is made up of hundreds of incredibly talented entrepreneurs, and even then, it's pretty evident that building a billion dollar company is as much about luck and circumstance as it is about talent and determination.
EDIT: Correction - I was very wrong about YC stats, as it turns out [1]. My mistake. I stand by the original point, though. 8 billion-dollar companies out of ~940 YC companies demonstrate clearly that building a billion dollar company isn't easy, and takes a whole lot of luck.