The class team, under Celestalon, bit off way more than they could handle with the revamp of all classes for Legion, and they're still finishing the work, and tweaking and adjusting constantly. They may reach a semi-stable spot right around the last patch - and then it all gets changed again for the next expansion.
Hey, it's job security.
It's instability for the players - it's a weird non-content that will keep people busy reworking their gear and such, but it's instability just the same. I'd hate to be running a raid team with the amount of changes that keep happening. But that's me, I always dreaded the start of an expansion, when they redid a class - I stopped playing my Moonkin druid because of how often they changed it. Just got tired of it all. But my rogue was always the same, I could blast out leveling as combat, and then go assassin or subtlety at level cap. Now....outlaw? Ugh. No thanks.
My favorite alts were always Rogue/Shadow Priest/Moonkin/Fire mage. I wouldn't have been very happy if I'd bought Legion, it looks like.