The biggest issue I would have with this process (which is the same for online [and offsite] testing) is that it could be done by someone other than the interviewer.

15-20 hour projects? I could think up something useful easily. At that amount of money i.e. 2-3 days cost it is a lot cheaper than finding you have employed someone you have to get rid of.

This is definitely a last (or penultimate) hurdle option. I like it.

Everything is self-evident to an author 😉