An electric heater's efficiency is "100%", but there are known ways to get much more than 100 percent for typical use cases. For example, heating your house with a heat pump can easily be 300% efficient, since you are also cooling down the outdoors.
"Electric heaters are 100% efficient" should really be "Electric heaters are only 33% as good as this other way of doing it".
"Electric heaters are 100% efficient" should really be "Electric heaters are only 33% as good as this other way of doing it".