The outright frauds are rare enough that checking for them needs to be very cheap for it not to be more cost-effective to spend that money improving other deal-flow filtering. That's not to say that VCs aren't trying to improve on it. It's just that fraud is just one of many risks, and managing risk is about taking a dispassionate view of which measures improve your returns the most.
I would also assume that the ideal investment would be zero due diligence in a unicorn. Any serious due diligence will affect the unicorn negatively, lowering end price.
I would expect a good unicorn to have strong antibodies against wasting time providing due diligence data. Maybe leading to an over-diligent VC missing out on the one investment they actually needed.
I'm sure the only thing separating Theranos from some other major companies that made it in the end is that they just couldn't get the product working, even after all the time and money they threw at it. If they finally had something that worked before everything came to a head, they probably could have parlayed that something. Holmes might have still had to account for something, and it would have been rocky, but really success is all that seems to matter, especially when a lot of people have a lost of money riding on it and don't want to take it as a loss yet.
Just look at the history of Uber. So many times they screwed up majorly, but they just coasted on through. Money invested provides a momentum all of its own to a company.
Would that be something akin to the "bystander effect"?