haha we've done several interviews like this and usually we pick a project that we estimate to be 3-4 hours and since we are engineers we pretty much always underestimate how long it will take. It's the first one we have ever finished / shipped. We thought it would be way too simple, turns out it was just right:)
We've had great experiences so far! Most small hacks / projects that we do end up turning into something that is honestly pretty fun and more often than not, we even end up working on it a bit more.