Blizzard has said that their goal is to make it so that all the covenants are equal and that you should choose the covenant based on which ability you like the most and which story/cosmetics you like the most. That's all well and good, but it comes with one fatal flaw. This has nothing to do with numbers and everything to do with the actual design of some of these abilities. Numbers they can tune, but it is highly unlikely that they will change the actual design of these abilities.
WoW has been going on for over 15 years. We know what types of abilities work in which content. That will not be changing in Shadowlands. If they don't make some big changes to how these abilities function, choosing one ability may very well make your performance in certain kinds of content very bad. I'm going to use a few classes covenant abilities to demonstrate this. I'm not going to post their entire tooltip as some of them are long, but I'll link to the wowhead page if you want to see what each does in entirety. Also I'm not going to go over every class. Just my main (Hunter) and the alt I play the most (Demon Hunter).
Hunter: https://www.wowhead.com/guides/hunte...es-shadowlands
Venthyr - Flayed Shot: Single target only which will make it bad for M+. Probably good for raiding and Arena PVP.
Kyrian - Resonating Arrow: Due to the flat percentage nature of this ability, it will be useful for both single target and AOE. Also has some potential in Arena although I think it will be pretty situational. Overall a good all arounder.
Necrolord - Death Chakram: This one is deceptive, but it essentially does the same thing whether on single target or AOE which means it is basically a single target ability that will always do the same amount of damage regardless of target count. Could be good for raiding or Arenas. Will not be good in M+.
Night Fae - Wild Spirits: Has potential on single target and AOE, so could be good for raiding and M+. Will be absolutely horrible for Arenas as you can either just walk out of it or just pillar hump the hunter while standing on top of it.
Demon Hunter: https://www.wowhead.com/guides/demon...es-shadowlands
Venthyr - Sinful Brand: Single Target with the ability to spread it by using meta. Could be good for Raiding and M+. Also potential in Arena as well. M+ usage might be more situational as Meta is a long cooldown, but could be really powerful as a 30% slow on all targets along with a powerful dot could be very useful on some pulls. Potentially a decent all arounder.
Kyrian - Elysian Decree: Basically only useful in Mythic+. Probably pretty bad in Raiding but might be okay. Will be very bad in arenas.
Necrolord - Fodder to the Flame: Could be useful for raiding and Mythic+. Will be absolutely horrible for arena. Like completely unusable. The enemy team just won't fight you where you can stand in the pool...
Night Fae - The Hunt: God damn this is a pvp ability if I've ever seen one. Will be very good for Arenas. Assuming you can mark raid bosses, could be good for raiding. Will be bad for Mythic+.
Now keep in mind I am basing this purely off of DESIGN. I am not accounting for damage numbers, buff/debuff percentages, or cooldowns. Blizzard can easily change those things. But unless they actually change how these abilities function, which they are far less likely to do when Shadowlands launches as opposed to numbers tuning, or they buff the shit out of an ability to compensate for it's weaknesses, which would probably make it OP as hell in the content it was already good in, then that's how these abilities are going to shake out.
Now as you can see both Hunter and Demon Hunter have 2 abilities that could actually be used in all 3 major forms of content with a decent degree of success. Kyrian for Hunter and Venthyr for Demon Hunter. For all the other three covenants for the respective classes, you are giving up performance in at least one other major pillar of endgame. Sometimes to an enormous degree like Night Fae for Hunters in Arena or Night Fae for Demon Hunters in Mythic+.
This is the real problem with the Covenant system as it stands. If I want to be able to do all forms of content effectively, then as a Hunter main, I basically have to choose Kyrian or I'll be gimping myself in one form of content that I enjoy. This could also complicate things with Hybrids where a covenant ability may be very useful for DPS but not very useful for healing or vice versa. Now maybe you'll get lucky. Maybe if you play Hunter Kyrian is your favorite Covenant and maybe it'll end up being mathematically the best. But the stars shouldn't have to align for you to feel good about your choice.
I also want to say that top raiders or arena players aren't the only ones who care about this stuff. It applies to anyone who cares about their performance in the game and considering that Recount is the second most popular addon and, if you add Skada and Details, which are Recount replacements, the number of downloads rivals DBM, then you'll see that the number of people who care about their performance is not a small percentage.