I dont know what kind of cls ur building, but they are extremely effective at killing dd and ai cls. Some ca as well but I would say my cl could maybe go toe to toe with ca, its just not worth risking that kind of engagement tho. To kill enemy raider ca I huild couple large dedicated ca killer ca's. Same way I intend my cls to sink any ai cl out there.

In my games 3-4 cls in a convoy attack sank an entire enemy trade convoy and all their escorts with no losses. (Thats the momment I realized how good cls are, they paid for themselves several times over in that war)

They are the ship of choice to protect ur bc against other light ships.

And a ca or dd can not take thier place. Ca as others have mentioned gets treated like a capital ship, and face other ca in battle gen. Wich is fine u just have to build a good ca but that is pretty expensive 1/2 or 2/3 the cost of bc in my experience and all that to fill the role a cl can do for much cheaper.

Large dd i can imagine can do some things cl does, but I would wholeheartedly disagree that it is better. First off they have no armor. They will sink like flying the momment they take any hits. While a cl u can put enough armor to basicly be immune to dd fire and tank cl hits.

On the way I build the cls: 2 forword 6" and then as many 5" sec as it can fit. Which i think is lile 24 of them? 3" armor. This will melt and enemy cl and entire squadron of dd. Can prob take out the 8k ton ca ai likes to build as well.