No its upsetting how glitchy it is. They obviously don’t care. Some people when they d/ced, NEVER got the mount and cannot run the event again on any toon. So… no mount for them… ever. You get it once and once only… and if it gliches away… too bad cuz youll never get a second chance. And btw there is supposed to be a pet which I did not get.
I’d say that is a bug, Blizzard will fix it.
There is no way to know it if it supposed to be account wide or not. Submit as a bug report but with a QA team shrunken in size good luck. 

Time and effort…really? For LFR? Seems like it’d just be tedium.
one thing that IS a bug is having the Obsidian Worldbreaker mount and the Lil’ Nefarian pet be REMOVED from your collections after earning them.
I never got it.
Yeah Blizzard keeps trying to give us the vanilla experience, this is one area where they keep succeeding at it.
When you hover over the Obsidian Worldbreaker mount icon it says Account Level Mount
The pet shows up in your mail box
i can’t evn do the stupid thing cause its a 2-4 hour que.
I just logged out and in, And now the mount is gone yet I have the achievement … bugged. as heck
Rofl that kid sure got into leet epeen mode. No pet. On any toon. Not in the mail, not in pet collection, not in the little box. And the quest to do the event GLICHED away from two of my toons and they are unable to reaquire it seeing as how the piece of paper that grants the quest went away as well. And the 15% buff… went away. It has a one time use and when you log, its gone. Not talking to you, talking… getting all upset and taking my post so personal. Goodness.
It will get fixed. Like to point out that the pvp mounts are per alt though. So Blizz does and can make it so that you have to do it on each character.
When you mouse over it in your mounts it says account level mount…so I am assuming its a bug.
For Horde on Illidan it is only an hour.
Many of the PvP mounts are character only, so that I get. The Deathwing one is the “general” mount for the anniversary though.
Oh this is crazy… earned it mounted up…logged of alt couldn’t mount on it …logged back onto my main… He is sitting on the mount… but its showing as unearned in his mount collection… GG Blizz … I assume this is being fixed.
Ditto. I logged out to hop on my horde and now the pet and the mount are gone. 
Wouldn’t be a bad thing though to make it per character.
Blizz could even have the same event every year for something to do.
It’s gotta be glitched. My belf opened his mail after my worgen main did everything and he did NOT even get a Lil Nefarion pet. Edit: The pet was greyed out at first but has since become useable by all. Just not the mount.