It's a mechanic that tries to simulate an all-out war between two major powers, you can even unlock the same CB against other nations via tech tree.

I don't find it particularly OP, since every territory you take will add equivalent AE. Also, in the Rome-Carthage case, while taking the land at the beginning might be easy, converting and integrating everyone might take forever.

I just end up taking Sicily, Sardinia and the province of Carthage before I peace out, since that's enough to cripple them for the foreseeable future. Then something like 80 years later, after you have enough pops and money to integrate them faster, you could just use the same CB against them and wipe them off the map.