They absolutely miss many big deals. Think of it this way, there are several dozen well-known Series A firms like Sequoia, Kleiner Perkins, Google Ventures, etc. But for any single company like Airbnb or Uber, only one or two or maybe three of those dozens of firms gets to invest. That means for any given unicorn, 90% of the good firms missed it, and probably 99% of all legitimate Series A firms. Sometimes that's because rounds are competitive and a company might have 5 term sheets but can only take one, but that is the exception and not the rule.

VC is an interesting business model because you don't have to get all of the good investments, you just need one or two per fund.

(Source: I'm a VC.)

There's a documentary about it on Netflix: "Something Ventured"