Exoskeleton robots have ushered in a new era for modern neuromuscular rehabilitation engineering and assistive technology research. This technology holds promise for improving the functionality of the upper limbs required for daily living activities. Despite the rapid advancements in exoskeleton technology, interdisciplinary research is still needed to address technical challenges such as kinematic compatibility and effective human-machine interaction development. This paper reviews the latest research developments in upper limb exoskeletons, highlighting key challenges faced in the development of these systems by comparing existing solutions. It classifies, compares, and provides an overview of the electromechanical design of upper limb exoskeletons. The paper summarizes current research on upper limb exoskeletons and discusses future directions for development.