Dragon Ball Super's foes were a cut above from the rest of the franchise as they needed to be formidable opponents to Goku and Vegeta's increasing God powers, and one of the biggest examples of this upswing was Zamasu.
The Future Trunks arc was one of the most well-received arcs in the series because of how strong Merged Zamasu was before he was finally taken down, and now series creator Akira Toriyama clarified a bit about just how strong he really is.
Regarding Zamasu's power when faced against two Super Saiyan Blue characters: pic.twitter.com/VTyD4WKsCI— Kanzenshuu (@kanzenshuu) January 3, 2019
In a chat between series creator Akira Toriyama and Dragon Ball Super manga illustrator Toyotaro in Volume 4 of the manga, the power discrepancy between Zamasu and Super Saiyan Blue's power levels were cleared up a bit (thanks to translation clarifications from @Kanzenshuu), "though immortal, his strength was such that two Super Saiyan Blues were more than enough to take him on."
This is important to note because Goku and Vegeta really only struggled against Zamasu and Goku Black because the two Zamasus worked in tandem. Super Saiyan Blue was a strong enough form to handle Zamasu at his standard level, and Toriyama originally designed him without much strength in mind.
It makes Zamasu's eventual Merged form even more interesting, and it seems various elements were added to the plot such as the reappearance of the Vegito fusion, and the Potara's new time limit, were introduced during the fight, "It's precisely because of [Zamasu's weaker strength] that in the original draft things unfolded so that his immortality and Potara time limit became key, and Goku and Vegeta took turns fighting him."0comments
With these comments, it seems that because Toriyama originally devised Zamasu as a character on par with a single Super Saiyan Blue the other elements were added to spice up the fight. It wasn't until Zamasu merged with Goku Black that he became a powerful god that pushed them to the edge. But before that, Goku and Vegeta were thrown off their game mainly due to Zamasu's tricky nature and further proving how their lack of teamwork at that point ended up being their undoing in the end.
Dragon Ball Super currently airs its English dub on Adult Swim during the Toonami programming block Saturday evenings at 11:00 p.m. It is also available to stream on Funimation and Amazon Video. The Japanese language release of the series is complete, and available to stream on FunimationNOW and Crunchyroll. The manga has chapters that can currently be read for free thanks to Viz Media. Tickets are currently on sale for Dragon Ball Super: Broly, which will be hitting theaters in the United States on January 16.