Slippi, in particular, has rollback netcode built on top of the emulator. It is way less laggy than Smash Ultimate and most other fighting games on the market, for that matter.

It is used for high level tournament play by players who have played the game in person since its release, with no complaints. It's really impressive how smooth it is.

I think you are right about general netplay. Some emulators are better about it than others, and fighting games are of the type where the lag differences will be especially noticeable.