I'm honestly sad that I have to admit that I've never played Smash Bros. Ultimate. I played a massive amount of Brawl, back when I still had gamer friends, but for the last decade I haven't really had gamer friends (some on PC, but not in-person gamers), and I never got a Switch. The only reason I'd get a Switch is if it had a Zelda game, and I played Breath of the Wild on a used Wii-U which I purchased specifically and only for that game.
I have no idea what I've been missing out on, I don't know any of the lingo, I'm not even sure if I'd enjoy playing it solo. I remember both Melee and Brawl having massive amounts of single-player content, which I loved, like different campaigns, challenges, and lots of unlockable that kept me invested in trying out different characters.
If the next Zelda game comes out on the Switch, I'll get one, and I'll catch up.