I've legit have matches where the game lagged in a way where it gave my opponent(s) the advantage and cost me the game, other times it came uncomfortably close to that happening. Needless to say I wasn't happy with either occurrences of it happening. Maybe absence makes the heart grow fonder because it's definitely happened back during online matches I had in Smash 4, but I personally feel like it was less frequent in that game than it was here.
I've also gotten the game almost always telling me that I can't do any Quickplays or Tourneys due to having an unstable connection if I've played online matches for at least more than two hours (if it wasn't for online lobbies, I'd effectively be barred from playing online completely), regardless if other people are using the internet or not. A design choice that Smash Bros. Wii U and Brawl never did.
Ragequittting an online match if you're losing or you've lost also seems trivial in this game because I've had countless online Quickplay matches which I won (or at least got second place) but I didn't get any GSP because someone checked out (mainly with 3P / 4P matches). At this point I honestly don't know if you even get any penalties for frequently leaving in the middle of a match (like how Smash 4 always barred your console from going online for ten minutes if you powered the console off).
I can only somewhat understand leaving during the middle of a match in the context of being knocked out during a 3P/4P match with a long time limit; so if you were the first/second player to be KO'd in such a match, all you can do is just sit there and watch the other players duke it out. Although in that case, there should be some sort of option to quit without having to shut off your console.
Ironically enough, for the first month or so, most of my online matches were absolutely fine and I didn't have to put up with any of this.
Still pissed we have to pay for online now and this nonsense only accentuates that.
EDIT: Before I forget--the only thing that I hate more than the lag is the matchmaking in Quickplay. Not so much the lack of options, but the way I get paired up with people who seem to get the exact specifications of stages they want so they can just cheese matches / get easy GSP by having the rules set entirely in their favor (playstyle approach, stage choices, items, etc.). It's like a lottery where you can get dealt with the absolute worst matchup for your character and your style of play. It's just barely enough for me to absolutely swear off Quickplay entirely and strictly go for Tourneys/online lobbies, b/c at least you can see the rules that are already set.