Dark is okay but it becomes noticeably worse as you progress. Former dark main who switched to thunder and it's worked far better than dark ever did. Dark just has so much shit that doesn't synergize, and so many himes who don't do enough...also some of the worst cooldown times/durations in the game come from dark mvps as opposed to any other element. Eg, samael (8cd 120s td def down), pluto (8cd 2 td dmg cut/atk boost). And dark is slow compared to most other elements because they lack data up abilities, you could always unlock their slots with magnas, but that means farming rags and you're new.