I think it was just too long and convoluted. There wasn’t a clear direction. The Cell arc was like that too—there’s the artificial humans, then another set, then plot twist! The real villain isn’t even one of them!
I think the downfall in the Boo arc is that there was no focus. It was supposed to be about Gohan, but then it shifted to Goku, but then Goku’s supposed to be permanently dead so shift it back to Gohan. Oh, but wait, no, it’s Goku. What’s that? He wasn’t the strongest? No, he JK’d, he is the strongest. He just wanted the “kids” to be the heroes since he was gone, but he’s not gone anymore, so it’s okay.
I also hate all the fusion stuff. What a terrible, unexplained cop-out.