Also to know is, what happened to Goku's arms?
During the Tournament of Destroyers in the manga, Super Saiyan Blue Goku uses this technique to attack Hit. Goku also uses it to counter Fused Zamasu's Holy Wrath and manage to damage him beyond regeneration, pouring so much energy into the attack that it in turn causes Goku's arms to go limp.
Also Know, who defeated merged Zamasu? Fusion Zamasu is defeated by Future Trunks. Kill him, and the Kais die as well, immortality or not? Gowasu, however, states that only Gods of Destruction can carry out that act. In the Broly movie, Gogeta fought against Broly and it was evident that there was a massive power difference between the two.
Herein, how did Zamasu become immortal?
Fused Zamasu An extremely powerful permanent fusion born between the union of Future Zamasu and Goku Black by the Potara earrings. Due to Black's body, able to power up constantly when in near death or while fighting, and Future Zamasu's immortality.
Why is merged Zamasu so weak?
The short answer to why Fused Zamasu was so weak(compared to Vegito) is that the fusing parts are no where even close to equal. Goku Black was weaker than either of the two Saiyans and Zamasu didn't have a snowballs chance in an oven against SS2 Goku let alone SSB.
