Autonomous systems are becoming more and more prevalent within industry. However, it is no easy feat to ensure their safety. Current safety approaches struggle to deal with the unprecedented levels of complexity introduced due to these new autonomous systems. More recently, System-Theoretic Process Analysis (STPA) was introduced to help solve some of these problems. However, real-world examples or practical reviews and guidance are still hard to find. To bridge this knowledge gap, this paper takes a critical look at STPA using the results of two case studies: an autonomous and a collaborative system. We present a loss and hazard list for autonomous mobile systems, alongside a more systematic method to structure certain steps within the analysis. Additionally, we reflect on the challenges that had to be overcome and highlight the differences between applying STPA on new systems, as opposed to applying STPA on existing systems. We highlight the importance of using the correct language / vocabulary and discuss how to build confidence in the results achieved by performing STPA.