Systems with moving parts undergo lots of stress. At some point, they will fail. Regular maintenance limits this; and the maintenance prescribed even for light aircraft keeps it to a minimum.

Still, no system is ever 100% reliable. The typical safeguard for engine failure in light aicraft is to glide to a safe landing. Starting with basic training, you'll do so repeatedly (starting from the vicinity of a landing strip, of course) and should be able to bring a plane down safely. Some (very) light planes even include a parachute system that'll bring down the plane in a manner safe for its occupants.

Speaking as a private pilot with little experience, this systems seems promising to me in two respects: In the case of a failed engine, it'll take you another 20 km or so. This can make the difference between a survivable landing in a field somewhere and a completely safe landing on the closest airstrip. Additionally, you get 40 hp or so of available power in situations where you need it, like taking off or going around after a botched landing. This could allow for shorter runways and help speed your initial climb, which is generally considered one of the most dangerous phases of flight: At that point you are both flying slow and low, and both speed and altitude are important safety aids. After a cursory first look, this system appears to be a considerable improvement in safety while avoiding much of the complexities a true twin engine system would entail.