Transformers designed for a 60 Hz that are operated on 50 Hz will have a lower input impedance when operating on 50 Hz power. Because of this, the transformer will run hotter and may overheat. Transformers designed for 50/60 Hz operation will have been designed to minimize the overheating. It costs money to do this, since more iron/and/or copper is required for operation at 50 Hz than operation at 60 Hz, which is why there are more 60 Hz only transformers on the market, and they cost the manufacturers less.
Bottom line: Both the equipment and the step down transformer must be rated for 50 Hz operation, or you run the risk of damaging the equipment. It may not happen, but then again, it may. :cry: