EPR measures the pressure ratio between the back and front of the engine. By measuring this accurately a precise measure of actual thrust can be measured.
As an engine ages the thrust will reduce for a given RPM because of tip wear, fan blade leading edge erosion, combustion chamber degradation and any number of other variables. However if the pressure ratio is X then the actual thrust is still X irrespective of how tired the engine is. Performance is therefore guaranteed.
Fuel flow is a pretty good measure of engine thrust but again, as the engine ages the EGT has to get hotter to create the same thrust as an new engine. Therefore higher EGT=higher fuel flow.
In twenty five odd years of flying EPR, have only seen two instances of a failure, neither on an Airbus.
Agreed the 'system' is not without fault....as the 737 into the Potomac vividly illustrates.