Togaroo,
"In the critical phase of take off wouldnt an early indication of engine problem be useful, therefore monitoring EPR would show a problem earlier than a reduction in fan speed N1 and allow direct action to be taken earlier?"
The best indication (although not the only one) of a potential engine problem would be a sharp rise / spike in engine vibration with a close eye then kept on the other primary engine parameters (noteably EGT).
The fan is the big driver, the one doing the lions share of the work and the reliability of its indication is (IMHO) better than that of an EPR system. On todays N1 system's there are no moving contact parts, with the probe end itself burried inside the engine where its nice and warm and free from ice. As well I'm with Gnd_Chk_Satis, its an easier system to maintain (Roller aside). As an example, the T700 on the A330 with the common nozzle assy has the nozzle EPR trimmed with the engine. If a problem is encountered with the nozzle, you cannot just replace it, it also has to have the correct EPR trim value as well. Makes maintenance all that much more stimulating.....
GE has got the right handle here.