I think that attempting to create 36 unique mini-classes was one of the biggest mistakes of Legion.
1. Development resource cost - 36 sets of talent trees, 36 sets of legendaries, 36 sets of artifacts, 36 sets of class abilities etc. Way too ambitious on the development side and the final product suffered with unfinished specs, imbalanced specs and almost non-functional specs. Too many development resources devoted to this and not enough to refining the game and acting on feecback prodivded by the community.
2. Opposite of class fantasy - this attempt at trying to make each spec unique has only made the situation worse. Why in the world does a Frost or Blood Death Knight not have Army of the Dead? The overall class fantasy has disappeared as class abilities have been split between the specs.
3. Balancing nightmare - I think it was quite obvious from the outset that there was no way that Blizzard was going to be able to balance 36 x 7 = 252 rows of talents, spec legendaries, stats, artifacts, etc. And they know this. At this point in the game's life span they should be trying to minimise the risk of class imbalance, not increasing balance problems.
I think that they should have stuck to the 12 class theme, kept the class fantasy, focused on balance and gameplay and really refined it rather than throw us this 36 mini-class gimmick for shock value. It really sacrificed gameplay.