It's a tricky problem, but ultimately you rely on the investigators caring more about their reputation than any one contract. If it gets out that firm X doesn't do objective reviews, then future companies in hot water won't hire them.

The real challenge here is that these investigators are not independent. Eric Holder has done prior work on behalf of Uber, which means he could reasonably be biased towards the company.

1. The public at large, who want to see standards of civic probity upheld? Or, 2. Future clients, who will want value for their money?

Delivering #2 requires writing up a designed narrative, in magisterial rhetoric. A narrative that hides all the dirt that isn't likely to come out anyway, and redirects culpability toward low-ranking dispensables, whether guilt or not.

For a big name like Holder, "folding up shop" would mean something like a beachfront retirement, in Malibu.

The independent directors on a company's board should hire the investigators, who then should report back to those directors.

Of course, in most startups, just like in many public companies, "independent" directors often aren't.