Im actually surprised they havent had an accident yet. The Harrier Jump jets were extremely dangerous because the exhaust would hit the ground and roll back into the intake and suffocate the engine. Pilots had to develop a special landing technique that was very rough on the aircraft. Basically a belly flop. Which is clearly not good for their frame life. So far the jetpacks seem surprisingly immune to this effect. But it would not surprise me if an accident were to occur eventually.