Well on a surface level, I think they're all pretty cool.
But if we're looking at this seriously, then I do agree with a lot of your words; fusion being brought up without any real foreshadowing (unlike say, Super Saiyan) in the Buu arc just so Toriyama can kinda….stall against Buu. Even Vegito, who was clearly stronger than Buu, was stopped by [plot convenience] then and against Zamasu. Gogeta kicked Janemba's ass but was written into the same Gotenks folly come GT. They're just too strong for the story so naturally they have to be stopped in some way. It doesn't even feel half as effective as it once was since a) Beerus shoved Gotenks aside and b) Goku and Vegeta will never agree to do it all the time.
In the outline he wrote for the Future Trunks arc Toriyama had planned for Zamasu to be defeated by Gokuu and Vegeta teaming up, not Vegetto. Vegetto was likely a decision made in part to sell Vegetto Blue merchandise or because Series Director Hatano Morio really wanted a big Potara battle, but I still think it would've been far more satisfying to have Gokuu and Vegeta use combination plays to destroy Merged Zamasu's Potara and then have Trunks kills Gokuu Black. Shida Naotoshi animating a little bit of Vegetto Blue was nice and all, but it's Gokuu, Vegeta and Trunks whom I'm invested in here, not Vegetto.
