Not to mention the fact that there is incentive for the people you are trying to use to try to mess you up. There are so many little mistakes you could make that would result in significant errors when added up that it would make the end result worthless. And it's in their interest to do that... the employees.
not necessarily. some (maybe most?) black box quants own their algorithm even if they leave the Hedge Fund. And of course they make profits while the algos are used. So the architects of the system are incentivized for it to work well.
