If you start a fire adjacent to an existing fire you almost always succeed, and the same should go for embers. In contrast, if you're unlucky, you may spend hours trying to light a fire in your fireplace (when there are no embers in it).
Embers shouldn't normally result in a fire, as you typically have to blow on them both to get them to increase to fire level, and to get through the ash that would insulate the new fuel from the embers if you just place it on top of the ash covering the embers.