I know Towellie raids current Mythic, but what exactly Asmongold do to earn this 'great at the game' title? All I see is complaining about game, doing some strange activities in Garrison and watching Bellurar videos. I think Pilav pointed in some old video that Asmon always want "more challenging content", but don't even touch what is in table right now.
I can see only two scenarious how this will go.
1) Blizzard REALLY screw up balance on Beta and just gives us way to use other abilities. Imo most elegant solution would be just way to swap covenant inside raid/dungeon/arena/battleground. Something similar to "mercenary mode" in PVP.
2) It will be more or less balanced (~5%). No, it's not like Blizzard always screw balance - for example 7.1.5 Nighthold was pretty well balanced. Of course I don't mean "!00% BALANCED".
Since SL will be less grindy, cutting edge players will make 4x more characters. Heroic/M+15 players pick something with good spells and won't care about covenants after that, but of course there will be "MAGE VENTHYR ONLY" tryhards in group finder. Casuals will enjoy vampire parties.