First example: Applies to loss of hydraulic fluid pressure however so caused. If there is no fluid, or the hydraulic pump is kaput, there is no pressure to drive the piston.
Second example: It's not the power piston getting jammed in the cylinder, it's the pilot valve that directs the pressure to the extend or retract side that gets jammed (because of a particle/dirt in the hydraulic fluid). Have a look at the Copterline S76 accident report.