Actually... Erebos shows perfectly what the problem with dark is. You get that very nice Vigor buff and... what ?
1. Pairing with Agaliarept or Dark Gaia doesn't work that well, because both drain party HP.
2. What are you gonna do to sustain high HP ? Use Osiris with that mediocre healing ? It may work... I guess. Use Nyanko-nyan to mitigate damage ? Sux that she works better when party is at low HP. Thing is that thunder can sustain with Ass-healer, Marduk AW and TAphro herself, water can sustain with Februus and Water Diana herself. Dark unfortunately is not very good at that.

That's the overall problem with dark, it does not have shitty Kamihime (you have Berith, Pluto, buffed Susanoo AW, Agaliarept, Satan AW, Erebos, there's a lot to work with), they just don't work very well together.