I don't think it is about challenging content alone that plagues covenant design.
Sepsis for example can not be used questing solo because it resets mobs as soon as the vanish is forced. Venthyr ability basically replaces poisons and isn't that powerful as it is a downgrade as you have to reapply it every 2 minutes compared to poisons that last one hours which makes it very cumbersome.
Kyrian Rogue ability relies on exact CP usage for max damage but it is hard to rely on that given how RNG Rogue CPs generation are in general.
The problem with covenants cuts deeper than top percent of players and min/max from my view.