I think this expansion has the best chance of any of having as little interruption as possible, barring DDOS which is mostly out of their hands as they're attacks on ISPs not Blizzard. The latest DDOS attacks on battle.net haven't actually affected me at all either, so I'm not particularly worried about them as yet. We'll see I guess. I'm on a lower pop server so not anticipating queues.
Four different zones, sharding tech, shared tagging so less bottlenecks, everyone being sent in different directions at first for artifact quests which are mostly scenarios, everyone who wanted a DH already has one so not even a mass of people there.