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.
95% could be accurate. I'd consider every "acqi-hire" an overt failure. Your business failed, but you personally got to fail upwards. acqui-hires are generally VC-engineere soft landings (everybody has friends they can ask for $50 million in a pinch, right?).
Just because money changes hands in the end doesn't make it a success. You fail upwards until you become a creature of pure thought leadership (i.e. a VC whose only job is to 'have an opinion' on stuff and can never be proven wrong because, hey, it's all an opinion or luck or the market or some unforeseeable outside force you failed to consider when failures continually happen).
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.
[1] http://blog.ycombinator.com/yc-stats