Would almost argue that Cell and Buu saga fumbled too, mostly in that it kept changing the 'big villain', and swapping in and out characters.
But I do agree that Goku/Vegeta repeatedly going back and fourth between time wasn't a great idea.
Although it's obviously because it was due to being rushed, I did like it ending on Zamasu just sort of becoming Giygas and very quickly screwing everything. It plays towards the idea of Shonen heroes gaining power from will. Zamasu genuinely saw himself as a hero. And when being defeated by the "villainous" Trunks, he wasn't going to back down — he'd triumph no matter what. With such insane mania and determination, that'd cause him to transform in what Gowasu claims to be 'justice' itself.
It's a very eery deconstruction on willpower, which I loved, despite it all being an ass pull.