The thing that annoys me the most about the online isn't the lag.
It's how it ignores your My Music settings.
The song that plays when going online is chosen by using the My Music settings of the person's rules that are used in the match. What's so baffling about this is this wasn't how it was in Brawl or 4 where each player would hear their preferred song on their end, so why is that not the case here? And it freaks me out that I don't see more people talking about it because this should be something that can be easily fixed in a patch.
And then you can't change characters in between games. Like...god. Make a separate mode if you want to take your GSP rank seriously. A "ranked" mode, as it were, instead of this "Preferred Rules" nonsense. I still get games of 1 stock 3 minutes, free-for-alls, 1v1 on giant stages, and even stamina. For Fun and For Glory had their flaws but they were reliable. You knew what you were going to get.
Smash Ultimate is touting itself as having the most amount of content in the series' history but I feel like it's never been more restrictive. I can't control the mode I play, I can't control the music I hear, and I can't choose the character I feel like playing without disconnecting from my current opponent.
Throwing my hat into the "well, MY online experience" anecdote pile, I'm living in the same condo complex I did when 4 came out, I'm using the same Internet provider on the same plan, and I'm used the same hardware for my Internet as I did with my Wii U. 1v1 is fine for the majority of my games but as soon as there are three or more players, the game instantly begins stuttering, whereas in 4 I played For Glory teams like 85% of my time with the game and had minimal issues. Hell, I had...alright...consistency in Brawl when doing teams, and I am dead serious. It might be because Switch is so much more popular so there's more players that just hop on without optimizing their connection settings or it might be because Ultimate has worse netcode or it might be because the Switch is worse at handling online, but playing Ultimate online with the lag, the lack of control over the music that plays, the lack of freedom over characters I can use and the lack of control over the modes I play, make it possibly the most unpleasant experience I've had with Smash.