I've never experienced an FF14 expansion launch. But I have experienced several WoW expansion launches...

MoP: Okay at first but then terrible once questing got going
WoD: Horrendous even with new Oceanic servers, laggy to high-heaven, quests bugged up the wahzoo, queue times stretching to nearly a full day
Legion: Smoothest ever, I was present for the actual switch from WoD to Legion, and it is still the best ever
BfA: Don't remember much at all about this one, but I don't recall it being terrible
Shadowlands: Somewhat between BfA and Legion in terms of smoothness

With the recent influx of players to FF14, I could only guess how Endwalker's launch will go. I do hope that it happens without major (or even minor) incident.