Maybe there should be a difference about attacking DD ability against subs and defending DD against subs.
It seems easier (because of the interaction with defending planes on carrier) to treat defending DD as OOB.
One DD cannot be everywhere.
In fact, 1 DD sculpt means many destroyers ships. And usually they are escorting other attacking warships.
So, they are not scattered around the sea-zone, every ship by itself, hoping to hit a submarine vessel.
I think that even a single defending DD is enough to block all attacking subs surprise strike.
I rationalize it this way: to provide a good defense against subs Destroyers just have to stay close to the others warships they are escorting. DD don’t need to be everywhere in a SZ.
On the opposite, when going Anti-Sub mission, they are patrolling a large zone and if they stay too close to each other it will be harder for destroyers to find targets.
That’s why I think the 1:1 restriction better fit for attacking DDs.
In addition, an attacking player always have the opportunity to ponder if he have enough units on his side to make an effective attack and, if not, he will plan something else.
If a defending player have only 1 DD with other warships, there is a lot of chance he will pick the DD as first casualty instead of loosing bigger warships.
So, in the second combat round, a large subs fleet will retrieve his first strike capacity.
Don’t you think?
About defending planes and DDs against subs.
What happen if, for instance, 5 Subs are attacking 1 DD+ 1 CV+ 2 FGs. Let’s suppose both Fgs get a hit but not the DD nor the CV.
According to the 1:1 ratio, only 1 Subs can be taken as casualty and the other hit is lost. I think it is OP.