A main aspect of Zamasu's motivation was seeing mortals as ungrateful and dangerous. Fighting against one capable of challenging or even besting divine beings would do anything but cause him to stray from that path.
Gowasu just allowed them to fight. The idea of getting Goku there wasn't his, it just happened as the Z senshi was beginning to add piece to piece. While he allowed them to fight, it's understandable thinking that having a physical engagement with someone described as unique may change his views on mortal. Besides, I don't even know if he vent to Gowasu his distate at mortals wielding powers in the God's vicinity. As far as Gowasu knew, he seemed to base such disdain on them commiting the same mistakes and not learning from them.
Actively going out and shooting down mortals >>> letting a whole resistance force stay alive for a long time when they were far from subtle in their actions.
He was incompetent in that regard, but I don't find too out of the realm of possibility to exist a small group that went under Zamasu and Black's radar. It has also offered good moments and given more weight to their quest of saving Trunks' world, so it wasn't that damaging.
He only outlasted him by revealing a bunch of asspulled techniques that neither Zamasu nor Goku showed any capability of knowing and are done away with as soon as they're used, not to mention Merged Zamasu never bothering to use such haxed abilities later. The manga at least stayed somewhat consistent in establishing his abilities and limits, as well as not wasting time on what amounts to just mindless spectacle.
Goku nor Zamasu developing thos techniques doesn't mean Black himself couldn't have pulled them out when cornered by Vegeta. It was also made quite clead that he came with those amidst the battle.
The manga doesn't squeeze such moves from him, but also goes for the more expected and boring route, without offering too much alternatives or variety for the character... whereas the anime seemed to be more well rounded in relation to Zamasu's motivation and Black as a whole.