Well, designwise they all.... work. Nothing more than that though. I've tried all my favourite specs but for legion I don't really find any spec fun. But yeah, they all work. Numbers are irrelevant for me, I've just been searching for a fun spec to play in legion. But I simply can't find that feeling you got when pulling awesome stuff off. Because it seems you can't pull awesome stuff off anymore.
UH DK's for example, no petmanagement at all, and the only dot they have left spreads on it's own. You really can't do anything that wrong but at the same time you can't pull anything awesome off either. Druids awesome starfall was ruined, and burst and aoe now require a long ramp up. Feral lost the feeling of swiftness when shapeshifting out of slows/snares in pvp, MM hunter got tons of abilties that all feel the same and hit for mediocre dmg instead of a few ones hitting really hard.
Assassin no longer have their awesome poison fantasy. Affliction have to stack UA which also feels really wierd in pvp. Spriest I havn't even tried tbh, looks stressful and not fun at all imo. Now this is all subjective and what I find boring, others might find really fun. This is just my experience so far while searching for a spec with fun gameplay for legion. Hopefully I didn't come off as negative as I think I did.
If I have to say something I liked so far, it's the heroic leap animation. That has some nice feeling to it, a feeling of awesomeness. Maby I'll just stay at 100 and heroic leap all day long like a boss.