[insoluble]

If by challenging you mean large and distributed, then I would agree these areas are probably less common in freelancing than in longer-term engagements. If by challenging you mean intellectually advanced, there are plenty of freelance jobs involving machine learning, computer vision, and other niche technologies such as robotics. Although my experience is more limited than that of some folks here, my guess is that most jobs, whether freelance or employed, are going to be run-of-the-mill. In many ways, computers were invented to ease boring administrative tasks, and this purpose lives on.