From a recent Telegraph article:

Professor Tyler Cowen, also of George Mason University, thinks that the problem of bad governance is far too complex to be solved simply by making predictions of how policy decisions may or may not turn out. "I don’t agree with the futarchy idea," he says. "The record of prediction markets is a strong one, but I wouldn’t want to use them to run an entire government.

Imagine a similar statement on voting:

The problem of bad governance is far too complex to be solved simply by having citizens elect representatives.  The record of representatives is strong, but I wouldn’t want to use them to run an entire government.

Or imagine similar statements about propositions, laws, judges, administrative agencies, public hearings, free press, constitutions, etc.  See the problem?  Every institutional mechanism is going go be far simpler than the complex problems to be solved – can that really be a reason to reject them all?