But... allies being worth it would basically mean that some armies have units that - without buffs because synergies are missing - are good enough so they are worth taking over own units that do benefit from buffs.
That's the critical point, because to me that means: if a unit from another army, without synergies, is better than all of mine that synergize with each other, then my army has a serious problem.
...and what kind of badass unit would that be? How would it fight in its original army, with synergies?
I see the point of allies, for specialist units.
I play a faction that has no artillery at all, or no wizards at all.
I can play pure lists, without artillery or magic, and those are still balanced in a way so I can win.
OR I can take allied artillery or a wizard, so I can use them as a tactical option.
But a whole detachment? Some line units and buffed up by their own heroes? I don't know.