To give an example Piper had to increase the HP on the PA31 Navajo from 310 on the non C/R model to 325 on the C/R model to get the same performance.
Did they have to, or was it that they were able to take advantage of the lack of critical engine in order to increase power and still achieve the 1.2 Vs1 certification requirement for Vmc? I don't know....
It is true that some twins have lackluster single engined climb, but they will have a positive rate of climb at speeds between Vmca and single engined best rate of climb speed. Or, they would not have passed certification testing.
Most light twins are not required by certification to be able to climb with an engine inoperative. They only need to demonstrate what the performance is.