They always had issues first days of new expansions.
I think Shadowbringers was by far the smoothest one... I don't remember too many issues outside of long login queues and some area crashes.
But a Realm Reborn, Heavensward and Stormblood were a quite a mess a times...

But even if there is a sudden influx of players (WoW), it shouldn't mean that everyone will still be there by the time the expansion hit.
Let's hope they don't add instances early n the expansion... I think they learned from Raubhan so it should be fine...