Installing UK/EU electric radiator in USA… 50/60Hz significant?
The resistive element in the heater will not care about 50/60Hz.  

The motor, however, will - to start with the different frequency will change its speed.  This in turn will change its current draw, to either less or more than it is designed for.  

This, however, seems like a fool's errand; resistive electric heaters are dirt cheap.  You will spend more shipping one heater intercontinentally than the heater is worth.  Buy your heaters on your own continent.  

The only case I can see it being useful to even think about the question is if you are fitting out a blue-water boat capable of an ocean crossing; or fitting exotic machinery unique enough to justify crossing oceans (say, in a container).  Or military equipment. 

P.S. Thermostats.  US vs EU thermostats work completely differently and incompatible.  In North America, the thermostat either switches the full 240V current, or switches a low-current, low-voltage signal.   In EU, the low-current signal thermostats use full mains voltage.   None of these three thermostats are compatible with each other.  Using a foreign thermostat is not wise, as it will be unsupportable - local HVAC guys won't know what to do with it.