Self-fulfilling prophecy: Believing that you will fall over because you are using clipless pedals can increase the chances of your falling over. I've never fallen over related to toe clips or clipless pedals. (Knock on wood.) Riding SPD style clipless for 20+ years.
I keep the release adjustment fairly loose so that it is always easy to unclip. If you wait till the last second to unclip, either on purpose or otherwise, you want it to be effortless. A little cleaning and lube once in a while can keep the mechanism in good shape.