The unique characterisation of characters seems completely gone. The cast doesn't blend as much as it tries to look like. Doesn't come natural like in games as the original, Melee or Brawl. Ultimate in general just seems like a project of how many characters could possibly fit into Smash. It definitely has its advantages, but this is a big negative for me.
Ultimate's engine is both great and terrible. It's completely unpredictable, and very aggressively offense based. Yet the neutral is completely chaotic and hard to predict. Whereas Smash 4 required some strategic foresight, I feel Ultimate does a step backwards in this direction. Melee does better in what Ultimate tries to achieve in this regard.
Online play is a joke. Terribly laggy as well. They should've learned to at least improve it by now. Think it's also a consequence of adding in so many characters as the main priority.
There's no reason honestly to play 1 Player Mode besides unlocking everything. Either add more modes to VS mode and focus mostly on the fighting aspect, or just leave it at mostly Ultimate's engine and focus on a more intense version of a worthwhile 1 Player Mode as a mix of World of Light, Subspace and Adventure Mode. As it stands right now, I feel that World of Light was a mistake. Resourceful, but not necessarily grand or glorious like even Subspace and Adventure Mode seemed to achieve better.
Items are stupid. Can't bear to play with them.
Many first party veteran franchises have been overlooked with regards to characters. Mainly, Donkey Kong, Zelda, Kirby, and previously Metroid, but that's handled now. These franchises are amongst the building blocks of Smash Bros., and it's this what makes me feel Smash hasn't respected it's roots too much. These sorts of new additions should be about as common as getting new characters from Mario and Pokémon. It wasn't Mario and Pikachu crossing over that made Smash as big as it is. Smash just took huge advantage of marketing Pokémon greatly at a very early and prosperous moment. But faces as DK, Link, Yoshi, Kirby especially also made Smash a strong first crossover for Nintendo. Otherwise it might as well have been a Mario characters Vs Pokémon crossover game.