I've my own theories on what goes on behind the scenes, but with nothing to go on, they are just that. I have tested both WiFi and LAN adapter. Most of the time it's fine. Every so often though, it is just not right to put it good terms. I hate when the match is going smoothly, I press a button and get the no response from the game. So my yoshi went from recovering, to ground pound, to forward smash, to recovering with egg throw, and just falling down to the stage, followed by a punish. I can work around it sometimes, but when your at last stock, both pushing 125%, just waiting for that perfect opening, that is when it really becomes infuriating. I use a Wii LAN adapter found at a thrift store for a dollar. Switch uses it just fine, it's WiFi funny enough is faster, but only by 1-2 Mb/s, so when I am getting a constant 30-36 Mbps down, and 7-10 Mbps up, I don't believe my internet is an issue, because my bandwidth on Fiber is 100Mbps down, 10 Mbps up, which is awesome considering where I live.
I don't wish to be that guy, but sometimes I do feel there is some jank behind the scenes. It could just be my bad luck as well. However you look at it, it gives online a bad taste for sure.