[RollAHardSix]

I wouldn't go as far as unethical but I will say it's a bit shady because it really does rob you of the chance to interview properly. Yes, if it is a hard real-world problem you're solving then it will look better on you, but at the same time, those types of hard problems are typically tackled by teams while an interview is more or less focused on YOU, and YOUR ability to work with a team. Or if you'd like I have a friend Frank who would love to come to an interview with me.