http://eu.battle.net/wow/en/forum/topic/17610511407#13
When discussing the Order Hall missions system in Legion, Ion (Watcher) Hazzikostas gives us some insight into the current state of the expansion:
This indicates that, functionally, they are still in the middle of Alpha. There are still quite a few systems to flesh out before we can even consider beta at this point:
1. Order Hall Campaigns
2. Adventure Mode
3. Suramar max level zone
4. Level 110 rewards structure
5. Improved social features
And once those systems are finally implemented, there is still a lot of fine tuning to do in beta:
1. Dungeon balance (Mythic, Mythic+)
2. Raid balance (LFR, Normal, Mythic)
3. Class balance (PvE/PvP, artifacts, legendaries)
4. Max level world content testing (Suramar, Adventure Mode)
5. Social features testing
6. Bug fixing
7. Stress testing
These are still quite a while away and Watcher's post on the official forums reaffirms this. I was initially assuming an August release but after Watcher's post and seeing the above, I am leaning more towards a September release now.