In recent years ontologies enjoyed a growing popularity outside specialized AI communities. System engineering is no exception to this trend, with ontologies being proposed as a basis for several tasks in complex industrial implements, including system design, monitoring and diagnosis. In this paper, we consider four different contributions to system engineering wherein ontologies are instrumental to provide enhancements over traditional ad-hoc techniques. For each application, we briefly report the methodologies, the tools and the results obtained with the goal to provide an assessment of merits and limits of ontologies in such domains.