The major "sudden" failure mode of pretty much all those systems has so far been the (brushed!!!) electric motor eating through its commutator (with the brushes still relatively intact) and stopping relatively suddenly - at that point, you'd better not ignore the lights on the dash, as it means there's only what's left in the hydraulic pressure accumulator in terms of assistance - afterwards there's almost nothing. The backup hydraulic lines have never really been an issue with this setup - the backup lines are two and directly go to resp. the front left and right calipers, and the only time they see pressure and fluid movement is during the brake bleeding procedure. The rest of the hydraulic circuits are somewhat safer as cars with such systems can absolutely detect and isolate a "misbehaving" circuit - i.e. if a flex hose is punctured, the rest of the system will work.