Rice cookers are the most commonly used tool for keeping rice warm because you are likely to use them to cook rice in the first place.
If you understand how a rice cooker knows when to stop the cooking process, it will be easier for you to understand how safe it is to leave a hot rice cooker.
If you need to keep the rice for more than an hour, don't leave it on the stove for that long. As long as the rice does not aggregate, you can leave it for up to five hours.
Leaving the rice at room temperature (until reheated) encourages bacterial growth
Never run a rice cooker for an extended amount of time without supervision.
Rice cookers are designed to keep food hot for a long time, so you can serve it when it's ready without the risk of overcooking or burning it.