Small 60Hz transformer work fine on 50Hz because they have a high enough resistance to limit the extra current and they're also designed for a slightly higher voltage, than the rating. Note that the power rating will be halved, since the maximum current will remain the same. Incidentally, it's fine to run a transformer at a much lower frequency, as long as the voltage is reduced proportionally, so a 240V 50Hz transformer will happily run at 25Hz, but at a maximum voltage of 120V. It's also possible to run at a higher voltage, than the rating, if the frequency is increased, but only up to a certain point, when hysteresis and eddy losses or the maximum voltage rating of the insulation become limiting factors.
In this case, a switched mode power supply transformer will be designed for operation at >50kHz, so won't be able to be used for any serious amount of power at 50Hz.