Well, you know I agree that Killer'a mental state didn't detract from his combat ability in any way.
It's the weapons that are the huge nerf, and I don't think it's at all comparable to Zoro using different sword.
You've shown example of Zoro using other swords, and being fine with them. Okay, that's fair. But fundamentally, those swords are much, much closer to what Zoro is used to using than what Killer had to deal with. Zoro was using one handed swords, they had a hilt, a cross guard and a blade all roughly the same length. Using Zoro's fighting style with them wouldn't be optimal, but it wouldn't be a great change.
On the other hand, this
Is a completely different weapon to this in every respect
The two bear as much similarity to each other as Whitebeard's glaive does to Shusui. The long haft, in particular, completely changes the dynamics of fighting with it, and even the blades aren't all that similar, one being far more curved than the other. Killer is used to hitting people with the blade starting at a great curve just on the end of his fist. With the scythes, there's an added length of wood to manpuever before getting in the position of hitting his foe.
I do not believe, for a second, that if Zoro was in the same position as Killer, and fighting with three spears or three axes- or perhaps going to the Davy Back fight and Muutoryu Zoro- that anyone would be saying that wasn't anything but a massive nerf.
Also to add to what you've asked on my profile, when I say the Luffy/Zoro and Kid/Killer dynamic I'm not comparing Luffy to Zoro, and Kid to Killer. I'm saying that Oda has been comparing the relationships between the two captains and right hand men- the Luffy and Zoro relationship is comparable to the Kid and Killer relationship.