I've had about a 90% success rate with these "Starters" (we used to call them ignitors). And I love the engine plugs that come with the engines. When I've had a mis-fire, it's almost always because in inserting the starter and the plug, the wires have made contact before the pyro, resulting in a short.
Take a lesson from the new starters for composite engines: put some insulating material up to the base of the pyro, so if the wires touch in installing, they don't result in an electrical short and mis-fire.