I'm gonna make a bold statement and guess that you either run Lu Bu + any SSR Debuffer and an EX for Debuffs or Lu Bu + Double Soul Debuffs, (maybe Nyarlatothep?)
That alone hinders you by Debuff %, the only way Lu Bu is feasible in an end game build is without Nyarlatothep (Since we're getting that C Frame Debuffer 35% now +15 then is enough) but as it stands currently the only sustainable way with it is with Amon Unleashed (-25 Def A Frame) and optimally running Vine for long content, but even then optimally you'll end up getting off by 20% Attack with Nyarlatothep and 15% with Hades or Trial by Jury...
That alone leaves up a lot to be desired, but then you realize that the healer Dark has is inferior to most element staple healers in Osiris and that hinders Dark even more and that's dismissing Black Propaganda, granted Dark has Beelzebub, yes but is that enough to counter all of this mentioned before?
To make Dark "good" you need two core SSR and to make it meta you need NY Amaterasu...
About Light, you have Belobog so there's no need for Raphael unless you really want that Orb Removal, Rage Down and Dizzy, Light Tsukuyomi would help you a lot but that would leave you in the same predicament you have on Fire with Mars but with a weaker attribute (Grid-Wise and Kamihime-Wise).
But then again a healthy end game account should have it's elements balanced and even if little by little a change needs to start eventually.