So now it triggers that in a APU overspeed (lets say during startup) if the APU has to be shutdown, you only rely on the EGT gauge (?)
For all practical purposes the NG APU monitors itself and automatically initiates a protective shutdown when it goes outside of normal parameters. The following are monitored and can cause a protective shutdown: APU fuel valve, DC powerloss, ECU, Fire, Inlet door, Inlet overheat, Loss of EGT, Loss of speed, No acceleration, No APU rotation, No flame, Oil filter, Oil temperature, Overtemperature, Reverse flow, Sensor failure, Underspeed, Overspeed low oil pressure, fuel solenoid valve fails open.
Those classics and originals how do you know your APU has overspeeded? again monitoring EGT? (in fact they have a red marking!)
Also has automatic overspeed protection, triggered at 110% by a centrifugal switch. EGT won't tell you anything about the speed of the APU.