To me, at least.
I do have to give the writers of Super credit for creating a full arc on their own instead of just extending (or padding, depending on your view) the two films that were the basis for the first 30 or so episodes. However, I feel like they wasted a lot by using Zamasu, period. Before I begin, I must warn this is a rather wordy essay (about a 5 minute read for average readers), so just bear with me, alright?
The first issue I have with Zamasu is that his character is supposed to be the "fallen god". In Dragon Ball, most villains are rather cookie cutter (the evil emperor, the lab experiment, the pure evil creation) which isn't a bad thing, but eventually, you would have to try other routes, and the idea of a Kaioshin going from a guardian to a merciless killer SOUNDS appetizing. But I felt the execution was hideously awful.
Zamasu's motivation was originated from a rather close-minded outset that all mortal life is a cardinal sin, and that all mortals are incapable of doing anything noble and heroic. It was like he "chose" this, rather than souring on creatures great and small for millions of years. Considering how ill-fit he was for this position, Gowasu probably should have pulled the trigger and hired somebody else (which, admittingly, does go against Gowasu's best intentions, but just because somebody has credentials, doesn't mean he's the best fit). To be honest, this idea I just said is nothing new; in an episode of "Bonanza", an aging Indian tribal chief kills his apprentice at the end of the episode because he was so consumed with hatred that he was willing to let his own tribe be killed just to seek petty revenge.
Then, you have what broke the camel's back; the spar with Goku. Goku, having trained with Gods and spent his entire life training for the future, easily defeats Zamasu in a contest. Kais are powerful, yes, but considering how helpless Shin was during the Majin Buu Saga, that wasn't exactly a red flag moment. Nonetheless, Zamasu decides right then and there that because a mortal ascended to God, all mortal life has committed the final crime and must pay with blood. Ironically, had he still existed during the Tournament of Power, the idea of a God of Destruction selecting a mortal to replace him or her would have sent him over the edge instantly.
So, Zamasu began a very complicated series of events. He used the Super Dragon Balls to steal Goku's body, which would, in theory, violate Zamasu's own code. I mean, for a guy who bloody hates mortals and believe they suck at everything, taking a mortal's body for his own gain seems counterintuitive (which they actually point out in Xenoverse 2). Also, if he wants all mortals dead, he's picking a rather slow and meandering approach to the process. The Saiyan Saga pretty much set the bar that ANYBODY could blow up an entire planet with a power level of at least 20,000 (and I know Super does away with that confusing metric, and that instance I mentioned was anime filler and removed from Kai, but it's still there). Killing mortals one by one is rather tedious.
Somehow, Black (as he's now known) makes it to Future Trunks's timeline and convinces the Zamasu of that timeline to join forces. So, this Zamasu wishes for immortality and the two now are wreaking havoc together.
Going back to the "fallen god" thing; I believe it's a good concept to explore. A great example I have seen was in the Injustice games. Superman is horrified that the Joker destroyed multiple cities and claimed the life of Lois Lane, so he sheds his beliefs and decides that the only way to protect everyone is to be more authoritarian than passive. However, this slowly morphs him into the very thing he originally hated, to the point where Superman is a greater threat to Earth than any villain originally was. Because we don't really know what Zamasu was prior to his promotion, really, we don't have that benchmark in which he falls off. And, on a slight tangent, the idea that he wasn't "evil" to Gowasu was merely due to Zamasu becoming, according to the archaic Dungeons & Dragons morality table, a "lawful evil" individual.
The second problem I have stems from my hatred of "time travel" TV shows. To me, this whole arc bordered on one of the worst anime I ever saw; "Buddy Complex". Buddy Complex was a convoluted mecha anime where a college couple are constantly flung through space and time (with convenient "amnesia" to boot) and wind up on opposite sides of a global conflict. The one who remembers has to convince the other her true identity, all the while dealing with a psychotic yandere boyfriend who is the main chain of this repeating history as his pure hatred for the main character. The hero is actually able to break this chain by saving his girlfriend and allowing the yandere to be flung back deep into the past. Unbeknownst to them, that yandere becomes a decrepit old man in their current timeline who now rules the enemy faction and wants to destroy the world... because he didn't get any. The hero and his friends finally kill the yandere once and for all and the couple is finally able to go back to their original timeline, which was just before when this mess began... only now the main character has amnesia. (Good news everyone; I just spared you from wasting 7 hours of your life as a public service, lol)
While the Future Trunks Arc didn't feature a 90 year old yandere who uses himself as an assistant for some stupid "Newtype" garbage, it was often a complete mess. The Android Saga did feature some time travel nonsense, but at least they kept it simple by maintaining one future with two different outcomes (Trunks kills the Androids, but is killed by Cell, and Trunks kills BOTH the Androids and Cell). Here, it was muddied due to how many different timelines were involved. Like, you have Trunks's timeline, which is the setting. But, you also have another timeline where everything is peaceful but goes to hell after Zamasu got spanked by Goku, and then the current timeline, where they succeed in killing the weakest Zamasu in the chain... and it ends with ANOTHER Future Trunks timeline where Zamasu is going to be dealt with becoming available. This meant we have four Zamasus running around, all of the acting like emotional wrecks complaining about the sin of man (I was also hoping for a more definitive means of dealing with the fourth Zamasu rather than a vague "We got an idea"). The fact we kept hopping from this timeline back to the present and then exploring the third timeline got confusing quickly.
The last problem I had was the resolution towards his defeat. Fused Zamasu was a very powerful, if not grotesque, final boss for the arc, but theoretically, it was his biggest mistake. Black was easily overwhelming the three Saiyans, while the only thing that kept Future Zamasu in the fight was the fact that he was immortal. By combining, the two essentially forfeited their advantages in both the anime and manga (where an immortal person could be killed, and a powerful foe had to compromise his strength with a weakling). And then INFINITE ZAMASU was born after Trunks slew the asshole, which was completely ridiculous. Like, how many Zamasus have to die to end this darn story? While I absolutely enjoyed his agonizing demise (but in a "Thank god this character is FINALLY dead" way rather than catharsis), it was completely unnecessary. This leads up to a rather uninspiring conclusion where Future Trunks & Future Mai decide to go to the new Future Trunks timeline, even though there would be some problems to that matter. I felt that, since they had nowhere to go, they should have at least stayed in the present, especially given how, more and more, the "Peaceful World Saga" & all of GT are looking like a separate branch of canon.
TL;DR: Zamasu was too complicated and too whiny to be an interesting villain. I'm not saying he should have been dumbed down, but a guy like him deserves a Kai treatment.
Thanks for those who read this little essay of mine and I'm interested in discussing this.