First of all, anyone trying to compare the covenant system to straight up classes is being silly. The only similarity is the somewhat permanent “choice.”
The issue with these abilities are the utility they provide. Mirrors of Torment is not even questionably designed for PvP. So if I want to Arena at an optimal level, Mirrors of Torment, based purely on the utility it provides, will be a must have, but the ability is almost certainly putting me at a disadvantage in other aspects of the game because the utility isn’t as useful. Frostjaw wasn’t particularly good in a raiding environment. Mirrors of Torment is Frostjaw on steroids. Why you do feel like restricting players to a specific aspect of the game is good design?