Why is this a problem? Have we not been looking for something that allows a realm with very few troops to gain an upper hand?
Think about this two realms, named A and B, are about to battle at region C. Realm A does an ambush and conceals 3k CS of their 13k CS total at C, and B has 9k CS. Now the smaller army that thought they would have a chance at winning or just a minor loss, got a huge loss. So even though it can help a realm with less troops it also helps realms with a more troops too so you can't say this a good feature just because it gives less troops the upper hand, it gives either side the chance a bonus. One issue with this is, other than no honor or prestige, there is no reason not to always put troops to ambush because it only helps the side that does it because nothing is bad about doing this in battle. So this seems like an overpowered feature.
