I think Unions and Industry Safety are kind of orthogonal here. Unions contribute to the industry safety of the employees, but I don’t think they inherently contribute to the safety of the customers.
Of course, this matters in the particular industry and role. A pilots’ union arguing for reasonable rest periods between flights certainly contributes to safety of both the pilots and the passengers. Nurses’ unions arguing for certain minimum staffing numbers and shift hour limits would also contribute to improving patient safety. However, steel workers’ unions arguing for improved safety rails above vats of molten steel probably doesn’t do anything to improve the safety of the people who use said steel, while it absolutely improves the safety of those who produce it.
Edit: Also, the airline industry is extremely heavily regulated for safety, even if you figure in regulatory screw-ups like the Boeing 737 MAX fiasco. Outside of union contracts, there are already tons of regulations related to pilot hours, maintenance paperwork, and so on, designed to protect passenger safety. In this particular case, I think the unions probably have little impact on passenger safety because government regulators already have all of that covered. Not that unions don’t have a place with respect to the industry – just that its place is probably almost entirely geared towards guaranteeing fair employee compensation and workplace safety, which I wholeheartedly support.