“Challenging” can also be considered “weird” or “unconventional.” My personal opinion about these types of essays is that colleges have grown tired of reading the same kinds of boring essay responses over past years and are looking for entertaining and insightful responses from their applicants.
Also, I think colleges are looking for original thinkers, young people who have a “voice” that they can express in an intriguing writing style. I would have loved to have had the chance to respond to some of the more unconventional essay prompts that have appeared recently. Of course, the University of Chicago is famous for its offbeat essay prompts. Consider these: