It basically exaggerates all of the game's inherent flaws to a silly degree, to the point normal interactions feel super awkward and full of guesses. Takes the reaction factor almost completely off the table, and this is where bad players thrive. They can "pressure" you for free and then mash panic options like crazy... or just anything, really. I play completely off of reaction, and I lose to players I'd never lose to offline (hell, I proved it). You can call it an ego thing, but it gets tiring. It gets really tiring losing to these god awful players. It's just common sense, and it grinds on your sanity at some point.
If you're not that serious into it, or your character, or playstyle allows it (ie playing "lame"), then wifi might be tolerable... at best. You realise the better you get at the game, the more wifi fucks you over. I used to be fine, or at least neutral to it when the game came out, but it's clear I didn't know any better.
And for the record, I use Joycons as well.