It was not a onesided fight like with Super Buu. Fused Zamasu was able to trade blows with Vegito and even kicked him in the guts and overpowered him for a moment, plus Vegito was pushed away by Zamasu's eye ki blast.Wasn't Vegito fooling around with him a bit in the first part of that fight? After the Spirit Sword dupe he starts kicking Zamasu's ass and mocks him for not being able to keep up with his speed.
Ultimately, Fused Zamasu's big mistake was giving into his darker emotions and expanding his size tenfold. Cell and Future Trunks made that mistake in the past too. What's the point of becoming a giant if you're just going to be pathetically slow and clumsy?
Even so, Fused Zamasu's broken regeneration was clearly getting on Vegito's nerves. After the Final Kamehameha miserably failed to annihilate him, Vegito was clearly irritated/upset and tried to finish off Fused Zamasu with that punch. Which, by the way, also failed miserably.
The very fact that Fused Zamasu was half-mortal in the anime is stupid. I don't generally praise the manga, but I much prefer that Fused Zamasu was completely immortal. I'm not really sure what was going through Toei's head when they decided that because one of his fusees was mortal, then Fused Zamasu had to be half-mortal but, spoilers, that's now how fusion works. It's not like Vegito was some kind of freak trapped between life and death in the Super Buu saga, he was 100% alive, even though one of the fusees was dead.I'm gonna give the Super the benefit of the doubt on this one because the story established that even when his regeneration stared to break down in some way in Episode 66, Merged Zamasu's regeneration capabilities were still broken enough that SSJB Vegetto still couldn't do anything to him and Merged Zamasu had to be literally cut in half down the middle to stop him.
Still, I understand the reason why Toei made that decision. While manga Fused Zamasu might be completely immortal, he is also much weaker than his anime counterpart (he was stomped by Vegito). If anime Fused Zamasu, who was able to hold his ground against Vegito, was also completely immortal, then he would just be unstoppable and the show would end because he would win.