“Why is Keiji one whole tier above Shingen?”
It is true that Shingen has better stats than Keiji when it comes to pure HP and DPS. However, Keiji is superior when it comes to mobility, attack rate, and foreswing. Keiji is way cheaper, hits extremely hard, has good attack rate, fast foreswing, can hit all traits, is wave immune, and has a tanker niche. Keiji’s stats aren’t even that far off from Shingen anyway while also having more targets to hit.
"Why is Yukimura still ranked so high even after Bahamut got explosion?"
Yukimura still has certain advantages over Bahamut, notably cost and cooldown. Both have and will continue to be used together or as alternatives to each other. Neither unit has any objective superiority over each other. Yukimura's build still lets him deal guaranteed burst in every situation.