The problem is that Blizzard has decided to add a whole bunch of stuff into the game without them even knowing how to balance the base classes and specs. For years and years there have been outliers, in every expansion, in every major content patch, that are either over or under-performing.
Now, they've added a whole bunch of other things on top of the base classes. Now that have to balance 36 specs under 4 different covenants. If we consider the covenant abilities and conduits under the whole package, that's 144 different builds they have to balance to within around 10% of each other (I always consider 10% being an acceptable difference in performance).
Then, you have to consider the different "modes of play": solo content, PvP, raids, and M+. Now, they need to balance how DPS performs under both single target, cleave and AoE, how tanks perform against trash packs and boss mechanics, and how healers can cope with huge incoming damage both single target and AoE, and what utility they can bring.
Blizzard has a lot of work to do, both prior and post-release, because we all know they're going to be doing a lot of heavy patching once the entire playerbase gets their collective hands on Shadowlands; this has been the case every expansion and it ain't gonna stop any time soon.