In the case of an all engines failure, the YEP can only be used if the APU generator supplies the electrical network. But, as per MMEL, the APU, the APU Generator and the Yellow Electric Pump are GO IF items. As a result, aircraft may be dispatched with one of these items inoperative.
Consider the cases of no fuel remaining (APU, even if operative, not able to start) or leak on the yellow hydraulic system subsequent to the Engine failure. In these cases, the flight crew will not be able to use the YEP in flight. As a result, use of the YEP in the case of an all engines failure would require additional system assessment and monitoring by the flight crew before they set the YEP on.
If fuel remains, if the yellow hydraulic system is in good condition, if the APU, the APU generator and the YEP are operative and usable at the time of the all engines failure event, then, the PTU must be switched off in order to isolate the green and the yellow systems. The YEP alone cannot pressurize both systems: The yellow electric pump characteristics (pressure/delivery curves) and the PTU efficiency are such that the YEP may just supply the Yellow and Green systems leaks if the PTU remains on.
However, even if the PTU is switched off, the extension of flaps is theoretically possible, but the deployment will be slow and probably periodically stopped due to interaction with the priority valve, depending on the hydraulic demand of the other consumers. The priority valve will shut off the supply to the flap motor in the case of low system pressure to protect the primary flight controls: As a result, the flaps may extend to an unknown position. The effects of such fluctuations of the yellow hydraulic pressure on the aircraft operation depend on many parameters and are not predictable.
If the Pilot Monitoring (PM) is distracted, and doesn’t manage to obtain the proper hydraulic system configuration, the aircraft might end up flying in a configuration with unknown results (i.e. YEP supplying both yellow and green hydraulic systems).
All engines failure event is a very demanding situation. During this short period of time, the crew's major concern is to attempt engines relight and/or to find a suitable landing field. Despite the potential benefits that could be envisaged from the use of the YEP in some very specific conditions, it is not worth complicating the procedure for the majority of cases and most probable scenarios.
For all the above reasons, Airbus did not opt for the use of the YEP in the procedures addressing the all engines failure situations (EMERGENCY LANDING procedure and ENG DUAL FAILURE procedure).