Let's be completely real here, other than that Kabito Kai wannabe who tried to fight against Super Buu only to be cock blocked by Old Kai telling him your weak go home which actually makes sense since he is a Potaro Fusion of a two garbage tier joke characters, Fused Zamasu by no means should be weak!
He is a Potara Fusion of Super Saiyan Rose Black and Future Zamasu who are perspective rivals to Super Saiyan Blue Goku and Super Saiyan Blue Vegeta in power, so logically Super Saiyan Rose Fused Zamasu should be a rival to Super Saiyan Blue Vegito in power correct? Wrong, not what we saw in the anime and manga adaptations of Dragon Ball Super.
In the manga version, the fight between Fused Zamasu and Vegito was described by the manga fans as Vegito vs Super Buu 2.0 because of how one-sided the fight was in Vegito's side similar to Vegito vs Super Buu fight.
In the anime version, Vegito had many opportunities to kill Fused Zamasu over 9000 times hadn't it been for Fused Zamasu's immortality saving him starting with the Spirit Stab to the chest.
To make things worse for our friend Fused Zamasu, in both version he was beaten around the bush/being bullied by a non-fusion opponents whom Fused Zamasu should have easily curb stomped in a fight. How is someone who is supposedly equal in power to either Super Saiyan Blue Goku and Super Saiyan Blue Vegeta was able to kill Super Saiyan Rose Fused Zamasu by simply absorbing a Spirit Bomb from a half-dead planet with only few inhabitants? A better question is, how is someone who is equal in power to either Super Saiyan Rose Black and Fused Zamasu was able to match a fusion of them both in power by simply mastering a transformation?
If Kefla a Potara Fusion of Kale and Caulifla was able to give Potential Unleashed Gohan and Super Saiyan Blue Goku trouble during their fight as a mere Legendary Super Saiyan, then there shouldn't even be a fight between Fused Zamasu and his non-fusion opponents, it should have ended with Fused Zamasu killing both Future Trunks and Goku by simply flexing his wrist!
For those of you people who are completely lost from what I'm getting at right now, think of a scenario where we replace Super Saiyan Blue Vegito in Super Saiyan Rose Fused Zamasu's shoes and Super Saiyan Blue Vegito is the one who ends up getting beaten around the bush/being bullied by the same non-fusion opponents using the same scenarios. How would you feel? That's how I'm feeling right now.
Fused Zamasu is such an awesome Potara Fusion character who has unique techniques and move sets unlike any other Potara Fusions, I hate of how Akira Toriyama did him dirty in both the anime and manga.