Most of the FM transmitters and receivers have a constant current consumption when in use, so placing a resistor between the between the 5-volt power supply and the module drops the supply voltage and protects the module. This can also be done with our OOK receivers, such as the LR Series. However, the current consumption of the OOK transmitters varies with the data bit, so they cannot be used with a dropping resistor.
This does have potential risks. The current consumption of the modules is normally steady, but it can change depending on how the modules are connected. It is best to measure the current consumption in the final design and size the dropping resistor appropriately.
The receivers output data at the lower voltage. The microcontroller or device that is reading the data needs to be able to recognize the lower voltage or there needs to be some sort of voltage level translation between the module and microcontroller.
While this does work, it is always safest to operate the modules with a regulated voltage supply at the appropriate level.