In order to spread warmth throughout a room, these electric fireplaces use a "fan forced heater". Large metal coils are heated within the fireplace, using electricity. A fan is then used to direct heat from the coils outwards, thereby heating the room. These fan forced heaters are silent and very energetic and efficient, allowing the heat to spread over a large area. As soon as an electric fireplace is turned on, the heat is provided instantly with no delay for heating elements to warm up.
However, it is not a big problem because you can always buy special cleaners for fireplace screens and glass doors. More often the construction of furnaces have special channels that send air flow along the inner surface of the door, and don't allow the soot particles settle on the glass. If your fireplace has no doors at all, you can use a variety of nets and shutters that can be installed directly in front of the firebox. They help to manage the air flow and thus regulate the heat level.