Hydraulic system failures are simulated in flight by turning off the hydraulics. This is not the only way systems fail, and one of the requirements in certification is to do a dry boost actuator flight. This often shows very different handling than just loss of pressure.
A Chinook in Canada had a hydraulic actuator blow out in one channel due to an unexpected failure in the AFCS circuitry. This put a much larger and faster hardover into one yaw channel actuator that was so strong it literally blew the side out of the actuator. The Chinook hydraulic system is duplicated, but the only thing between the two systems was a rubber O-ring. This was deemed sufficient because the failures predicted had said there would always be fluid on both sides of the O-ring, even if there was no pressure. Alas, no fluid, no safety and the pressurized fluid on the other side of the actuator blew through. Massive hardover in yaw (helicopter was reported to be rotating in yaw at 180 degrees per second).
Lots more stories about AS-350 hydraulic problems are out there.