It likely is the biggest outlier, even beyond M+. Door of Shadows is like a personal warlock gateway that has even more flexibility, and gateways were used constantly as the de facto strategy or capable of trivializing certain mechanics throughout the BfA raid tier, and previous expansions prior to it. I still remember mythic G'huun: the difference between having 4 warlock gateways versus 3/2/1/0 was huge when tackling that encounter. Add on top any mechanic that requires people to travel long distances quickly, whether to avoid mechanics, get away from the group, drop mechanics, etc... Door of Shadows is super useful for this. Soulshape is a decently close second, the rest aren't even close.
While people think this will affect only the 1%, it truly does hit everyone. Irony is that while people think that using something like Door of Shadows to skip trash packs or create shortcuts only applies to +25 to +30 keys, you have to keep in mind Door of Shadows solve a difficulty problem relative to the player skill/ability. While that trash pack may be deadly for a highly skilled group of people on a +30 key, that trash pack may be just as relatively deadly for a group of average people on a +9 key. The relative benefit of everyone having Door of Shadows is the same despite the difference in the difficultly of content and the skill of the players. In some respects, I'd say it's even more beneficial for the lesser skilled people on lower keys who may need every bit of edge they can get.
Similar thing applies to raids, as well: the relative benefits are what matters. Having Door of Shadows with a CE guild on normal raid difficulty is obviously not going to matter, but for a friends and family guild that may not really have great skills... it could make all the difference in the world if Door of Shadows allows them to trivialize mechanics.
Abilities that allow people to trivialize mechanics are the main issue here when it comes to covenants. While an extra 1-2% may matter to some degree, even CE raiders aren't concerned about that. However, the benefits of an ability like Door of Shadows is so varying and universal across all content and just too good to pass up as things stand right now, if you ever intend to hit a content wall where your skill may come into play. Only thing that would be more obscenely broken to introduce as a covenant ability would be an immunity, and some of the cheat deaths come close. One was already removed, and I think the Night Fae one only exists because it has a 30min CD right now.