The abandoned tests highlight the difficulties in balancing the costs of robots against their usefulness, said Matt Beane, an assistant professor in the technology management program at the University of California, Santa Barbara. Robots are still prone to messing up basic tasks and often require supervision in fluid environments like shop floors or sidewalks, Prof. Beane said. Their success in public settings has been largely limited to high-margin businesses in contained, predictable spaces—delivering room service at luxury hotels or mixing cocktails in bars, for example—where robots are often novelties for which companies can charge extra, he said.