This situation came up in the tournament last month, the first time I had come across it, and the situation was as follows:
My large infantry unit was charged by a small cavalry unit, and so formed square.
The charge came in on an angle so that only one end base of the infantry unit was contacted, even though the centre bases were 'covered'.
There was an artillery unit nearby, which took the opportunity to fire on the exposed end of the large infantry unit at +1 to hit because they were in square, promptly dropping them a cohesion level before combat took place.
All of this is according to the rules, and I have no issue as far as that is concerned.
But it just doesn't
feel right.
It seems to create a vulnerability in large units that can never happen to small ones - when faced by a large unit, position your charger to come in on a slight angle and then have a chance to drop the assault target a cohesion level through shooting
before combat, with very little risk of the shooter being hurt in return - with that shooting unit then possibly in position to provide flank support as well

.
This is bad enough for infantry vs. infantry or cavalry vs. cavalry, but for cavalry vs. infantry it gets worse with the shooting POA.
Until this all came up, I had always assumed (incorrectly!) that shooting at or by units in combat was not allowed. This was something that happened in FOG:AM or R, but those rules have much larger units, and step forward their assaults - both of which make a difference.
It seems somewhat unfair that a large unit suffers from this, while a small unit will never be affected.
And it seems to encourage the sort of micro-management of angles, and allows part-unit shooting, that these rules try to eliminate.
My preference would be to keep things simple - no shooting into or out of a combat.