Personally, I don't really care for the, "Everyone is here" aspect of Ultimate. I'd rather have more new characters than a bunch of old ones. I can't say for sure that we would've gotten a ton of new characters if characters like Pichu & Young Link weren't brough back (since creating brand new characters requires way more effort than just bringing back old ones), or if characters like Falco & Roy were cut, but if they were removed so that more development time could've POTENTIALLY gone towards brand new characters, I'd be all on board for that.
The same goes for stages. It's nice to have all of these stages back, but only having 4 new ones is kind of lame. I can live without stuff like Hanenbow, 75 m, etc... If it meant more brand new stages (once again, I'm not saying that we would've gotten a brand new stage since those are harder to make than bring back old ones, but if there was even the slightest chance... You get the point).
I'm not sure if this is unpopular, but custom moves are way better than spirits. The weapon triangle thing for spirits is dumb, and makes beating people nearly immposible when you do 0 damage to them, but they do 33% with regular moves like n-air. Custom moves were broken, but they weren't that broken.
I never understood why people got called out for not wanting to get Ultimate because their favorite characters didn't make it. To me, that's a perfectly valid reason not to buy Ultimate. It's not like the game is free. If that person isn't satisfied then they shouldn't be called things like "childish" for not wanting to drop $60 on a video game.