Best not to assume dual voltage - many cheap ones aren't. Some are. You really just have to look at it.

The frequency of the power isn't important for a resistive load like a heating element. You just have to worry about voltage (well, and current, but in this case just voltage really).

As for the reviews saying it doesn't work - I suspect those people didn't understand how to use it correctly. As it says in the description, you set the switch to Low for devices that need less than 25W, and to High for devices that need 26-2000 W. Set it to high and plug in your curling iron. If the curling iron is dual voltage, set the curling iron to 230V and just use a plug adapter.

Many international hotels now have dual voltage sockets in the bathrooms as well.

This really doesn't have to be so complicated!