I have some experience with Sorceresses.
What I've found so far :
There is one specific case where Sorceresses can wipe the field of the opponents - if you field all Sorcs against 5 or fewer stacks of ranged (light/heavy) opponents, with no melee.
Specific case - I put 5 stacks of Sorcs against 3 stacks of Bandits, plus one of Cannoneers - normally a terrifying combination, because 40+ Cannoneers can take out a single stack of Sword Dancers in a round, depending on luck, and if the terrain blocks my way, I may have to survive 2-3 rounds just to get to them.
In this case, the Sorcs started out of range of the Bandits, and the Cannoneer didn't kill too many with their first attack. After that, each Sorc unit attacked a different enemy stack - their -60% defenses against ranged attacks stacked with the Broken Valor power (-60% to attack), and none of the enemy units could hit any of mine. I wiped out a challenging group with no further losses.
However ! This doesn't work if the enemy has
more than 5 stacks - in that case, in every round one of the enemy stacks will be able to attack, and in sufficient numbers they will quickly whittle down your army. (I learned this one the hard way, just after the previous encounter
)
Also, Sorceresses don't seem to combine well with Treants - e.g., depending on the sequence of attacks, something like this might happen :
Sorceress hits nearest enemy stack, they now have -60% on their attack.
Treant II hits the same stack, the Daze effect applies and
overrides the Broken Valor, now they only have -20% penalty !
Then the enemy stack gets their attack, at -20% (not -60% or -80%, as you might hope for).
Finally - the Broken Valor
icon which appears on the enemy units seems to last longer than the
effect - i.e. I've had units with the icon still present attack, but the hit/damage figures show no -60% effect on them.
So, in summary:
They can be useful, they work best when mixed with your Sword Dancers, assuming the SDs can block any melee units from the Sorcs.
Obviously you need to keep them protected, especially if there are Cerberuses in the enemy army - they can wipe out Sorcs very quickly.