I once marched a mercenary unit from Sasrhas to Osaliel and back. Equipment damage was a big problem, but morale was manageable.
I have twice marched an
army unit from Sasrhas to Taop and remained there for weeks without even the slightest morale drop. The message was pretty similar to "your men complain about being far from home, but their morale rises anyways X points".
So, army folks can stand long distance campaigns without penalties, but mercenaries can not? Even if there is only a reduction of the penalties, and not having them disabled, the bonus should be MUCH bigger than it seems to be right now.
They are mercenaries, not crying babies, for god's sake.