Are the advertised weight capacities on garage shelves accurate?

Generally no, at least not in the way buyers expect. Stated capacities are typically total across all shelves, often measured on level concrete with evenly distributed loads, and specialist-community threads on r/garageporn and r/Flipping repeatedly warn that thin uprights flex well before the rated maximum. Treat advertised numbers as marketing ceilings, not working loads, and derate by 30–50% for real use.