Completeness doesn't imply that an element is OP compared to others. For instance, consider death's success in war: death doesn't have any PC or protection! On the other hand, darkness, a very complete element, has never won war. (NOTE: This is NOT to take anything away from team darkness, or to make a stab at them. I just say this to show that darkness is not OP).
Also, an element doesn't exist as an island; not all decks are monos. Therefore, if an element that is devoid of a certain type of card can splash another color, or run more complete duo. For instance, light lacks CC, but it has synergy with air, which does. Also, light + aether is fantastic for stalls, and also has CC. Furthermore, completeness does not imply that an element is OP. Rather, that lack of completeness can allow more cards within an element to be directed towards a singular purpose, such as life gain/stall, rushing, mass CC, etc., which makes it stronger in one or two of those individual areas than an element that can work in every area. It's like in racing games that we played as kids: the driver that was decent in every stat wasn't always the best driver to pick. Some courses with lots of turns required a driver with lower speed and better handling, while some wide-open straight courses favored drivers with higher speed and worse handling. I mean this, of course, to relate to deck match-ups. While darkness is a fantastic element, I'm sure we can agree that there are non-darkness decks that can rush faster or stall longer. Darkness CAN do both, and can excel in all styles of play and deck creation - and that's its fantastic strength - but it does not mean that this strength is OP compared to the performance of other elements within their niches. All in all, the strength of an element does not always rest on its completeness, but rather, on other factors such as synergies, and niches.