Spent a huge amount of time (more than a year) working alone in a face detection API. The hardest parts were 1) marketing, how to make business know it existed and 2) keep up with the state of the art research implementations. The 2) was not difficult if I had had at least one client to pay/motivate me to keep up with the development. It is still on at www.rosto.io but I am not actively developing anymore. At least I have learnt some stuff tech-wise.