I think the concern is that the homework task could be too large and because people have an incentive to appear competent, they might lie about that to make themselves look bad. That means that we're giving too hard a task but we'll never find out.

As to how likely it is that people will lie, given that we've hired several people who did this homework and they've proven to be as competent as we believed, there's at least some anecdotal evidence that some people have not lied.