Black did explain that the more he gets hit the more his power grows because he gets to fully master Goku's body or something like that... but I agree it should've stopped long before he pulled off that retarded scythe they only used in that one episode. The manga tried to make it less cheap by requiring Zamasu to heal him but it was still garbage because he surpassed SSJB even without Rose in just one boost lol.
Zamasu and Vegeta's power ups aren't thaaat bad I'd say, Zamasu still had over 15 years to train compared to the present Zamasu while Vegeta went in the Rosat, what's really to blame for their ridiculously inflated powers is how everyone's performances tend to depend on plot convenience in the latter half of the Trunks arc (Trunks overwhelming Zamasu without SSJRage, Vegeta and Galu somehow holding off Merged Zamasu despite an ocean of difference between them).