Responding to this to try to clarify my comments:
I didn't mean that Egypt isn't controllable, just that it seems as far as the state can exert control when things get unruly (not disputing that it was more a matter of people stirring up trouble artificially than the Egyptians not accepting Persian rule).
Controlling something as far away as Carthage which was giving an equal level of grief would be too much, thus what you said on not going further.
Oh, of course. i must have misunderstood something. As I said before, Carthage is too far. Gandhara was even a stretch, which is why it was lost in Xerxes' time.