Criticism has centred around two points in particular.
First, the general lag which can crop up when playing any of the game's online modes. The Smash Bros. series has consistently faced problems with this when playing online - but this isn't the Wii era anymore. As a fighting game, any lag at all can kill the experience and ruin any sense of fairness when playing competitively. And sometimes, seemingly for no reason, the game chugs down to a near-total stop.
There's the added criticism here that, with Switch, Nintendo has introduced a monthly fee to play games online. For it to take money and still provide the same uneven service you sort of excused because you were getting it for free has unsurprisingly got fans grumbling.
Second, is Smash Bros. Ultimate's frequent inability to find a match with the settings you want. This is particularly true for those who just want one-on-one fights, or matches without items. Even if the majority of players want four-player battles with items on, Smash does not have a small player base and fans have questioned Nintendo's own netcode for continually failing to connect players.
Super Smash Bros Ultimate really is something special, man.— Leffen (@TSM_Leffen) December 9, 2018
In what other game can you have a 4 man match where Cloud, Joker, Mario and Sonic face off, with pokeballs and assist trophies going off left and right, despite those 4 players all searched for 1v1 no items.
Nintendo has yet to address the criticism, but has today released this notification in-game promising the game's first post-launch patch next week.
Will this contain improvements for how Smash Bros. Ultimate prioritises player preferences when matchmaking online? People hope so. Right now, Ultimate's online portion is not living up to the rest of the game's polish.