Fuel exhaustion, starvation, fuel contaminants such as water, catastrophic failure of components that are operating under extreme heat and stress, carburetor icing caused by high humidity, engine component icing (pressure/temp sensors on turbine planes, air induction icing on piston power planes), bird strikes. Piston and turbine aircraft engines are phenomenally reliable but all the best minds and all of the money thrown at the problem over the past 112 years have not reached a 0% failure rate. Electric motors don't have the complexities of combustion engines which would be reassuring when flying a single engine airplane over water, in bad weather, or over a populated area. I think it's an awesome idea that could add a huge margin of safety relative to the cost.