As a hirer, the kind of takehome assignment I like to give is one that:

* Can be completed in 30 minutes by a skilled programmer

* Has clear evaluation criteria, both objective and subjective

* Has multiple approaches that require making different tradeoffs

And of course, only give it to some candidates where the result will be make-or-break.

As someone who took one of these broad take-home assignments my last time looking for a job, I failed a the assignment for a job I was overqualified for because I was told I wasn't able to divine what parts of the extremely broad assignment I would be graded on.

I doubt I will be in a position where I get a job that isn't a referral for the rest of my career, but it really turned me off of these kinds of assignments, both taking and giving them.