what are the. operational. differences. between FADEC and HM FCU??
One is electronic and the other mechanical....
A FADEC system senses engine parameters (e.g N1, N2, Pc, Temp, Tq), as well as pilot demand (power levers) all electronically (transducers, tachos, LVDTs, etc) and a 'computer' or EECU (engine electronic control unit) does the maths and tells a metering unit how much fuel to give the engine.....without over-limits.
A hyd/ mech FCU senses speed (e.g N1, N2), usually by shaft driven fly-weights, and along with using 'fluid' mediums such as Pc air px or fuel px, mechanically governs the amount of fuel the engine requires...all this mechanically coupled to pilot demand. The avoiding over-limit part is more of a pilot responsibility here, as there is no computer involved.
Get a good GT engine theory book such as Rolls Royce or Jeppesen and all of this will be clearly explained.