The 240-120 voltage problem can be remedied by using transformers but that doesn’t negate the frequency difference between grids. Australia operates on 230v @ 50Hz while the US works with 120V @ 60Hz for instance, as certain electronics that are singular voltage (Hair Dryers, Desk Fans or Electric Blankets) have their frequencies disclosed even if the voltage on those are different than the one that country uses.
Does plugging in a device that has 120v-60Hz via a transformer on a grid that operates on 230v-50Hz ruin the device due to either heating elements or motors inside? Even between countries that work with 220-40v but one uses 50Hz while the other uses 60Hz, does that render those electronics (like dehumidifiers, power drills, curling irons) incompatible despite them using similar voltage (like 220v vs 230v)?
Japan is split and has both 50 and 60hz grids. Modern products don’t have any issues with this (voltage is 100v typically). In the older days, this was an issue for things with clocks and motors from what I understand, but I didn’t move here u til the 2010s