Legion was a very, very good expansion from an overall perspective. But it had a lot of problems. And it was VERY buggy at release. There was tons of DDoSing, problems with M+ key bugs, problems with quest bugs (anyone remember trying to start aszuna and then having Khadgar just disappear when you landed and even abandoning quest and trying to restart didn't fix?), problems with class bugs (remember disconnecting every time you tried to leave your order hall as a warrior?), Bad Luck protection actually being Good Luck Enhancement at first, Queues getting stuck at 1 and never moving, Having to clear your battle.net cache so you could even hit the "Play" button on the launcher, the reintroduction of loot lag, etc etc etc.
A lot of people are comparing it to WoD, which I didn't experience so I have no comparison, and I guess if WoD was that bad anything is better? But Legion was the worst launch I remember since Vanilla outside of that and MoP and WoD are the only launches I didn't experience. TBC, Cata, WoTLK were all much better than Legion from an overall bug and stability perspective. I think I may be confusing actual launch vs when Mythic + and Mythic raids were released in Legion as far as DDoSing and not being able to login or actually play, but there were a lot of problems with Legion.
I really hope the worldwide release doesn't affect our ability to actually get in and play tomorrow.