For sure it was and by far before haki was introduced since you get a free pass tobthe logia user body without amy need of a trick (water for Croco, rubber for Enel etc.) but the inroduction of haki almost destroyed his main perk (at elast as far as we know).
Yet the point for me is that the yami has very strong perks (maybe the yami is also the key to get 3 df, most likely there is a reason if BB targeted this specific df for decades, also gravity is not bad at all and it can always nullify other df attacks like with WB gura hit etc.) but also great downfalls (like the increased damage taken) and now we go back to the Kaido df theory. That df gives Kaido an almost total invulnerability and that will cover perfectly the yami weakness plus if he can grab a logia (like the original admiral trio) like he did with Ace and deliver a quake punch in hybrid dragon form that would be a lethal attack. They should synergize very well and maybe it is not a random fact.
There are some theories that suggest he will get the phoenix fruit of Marco but that gives only regeneration, an almost total invulnerability is still better.