Please note the reason i post here is because i think these two issues are the biggest issues with SSBU & i don't think they get talked about enough.
honestly my main complaints are
1 many of the playable fighters' move-set(s) make a disastrous & ineffective play-style.
2 My other big complaint is most of the ( arenas / stages ) are not fun to play on.
numbered Examples
1.1 is how Yoshi's Side Special is quite fully useless and generally does not improve his gameplay.
1.2 Corrin, Belmonts, Bowser, Ridley, & ZSS, all have a down air which if used incorrectly results in a SD / lost stock.
1.2.1 Bowser, Yoshi, all have a down special which if used incorrectly could actual result in a SD / lost stock.
1.3 Zelda's forward air being useless 98% of the time makes her a snooze-fest to play even powered up with a powerful spirit.
1.4 these characters' move-sets really need an overhaul. ( Samus / Dark Samus & ZSS, Peach / Daisy, Pac-Man, Mario & Luigi, Ganondorf & Link )
1.4.1 these characters' move-sets really need a buffing update ( Zelda, Mii Gunner, Bulbasaur & Charizard, ? )
1.6 The heavies are bottom tier for sooo many reasons the three biggest are; ( they don't have ranged moves or their ranged moves lack any meaningful element of surprise ) ( Heavies' ground or air movement speed or both is usually strongly related to molasses. ) ( Heavies' recovery is terrible. )
2. generally the platforms are either to small to be useful or poorly placed. or the hazards are miserable & more reasons too not listed
2.1 Battlefield ( regular ) has 1 too many platforms; the top one
2.2 Hyrule castle ( tower portion )
2.3 Kongo Falls some platforms are too small
2.4 Jungle Japes the hazards are miserable
2.5 Brinstar the hazards are miserable