Shoud be a requirement on every amusment/thempark ride that use lapbars. May accidents (both mortals and non mortals) happen because of the lapbar not locking the person properly. Either the person is to fat or to small (kids).
Impossible, even if one failed the remaining one is strong enough to hold a 500 pound human locked in the seat throughout the ride. And for a hydralic system to fail, you need a chain of events to occur, manufacturing error, installation error, maintenace error (both the yearly revision and routine inspections) and then maybe you will have an accident.