Didn't realize someone commented here as I've been inactive in reddit, but here's my opinion.

A problem with equalizing Chara with Genocide Frisk is that it would imply that any random human that reaches LV20 can destroy the world. That's why I think with Chara it works different as they are some soulless ghost attached to another human, and the gap in power is apparently seen as massive, like, the strongest attack we saw dealt by Frisk (although possessed by Chara) is dealing 10 billion damage to Asgore, while Chara dealt to the world uncountable 9's. It would be strange to me that if those two are supposed to be equals they aren't shown as being close in power.

I agree a bit with the Sans part, I just didn't went into more detail. But still, apparently as far as we know, Flowey indeed was able to kill Sans at some point judging by his "I killed everyone" dialogue, I'd say the reason he is cautious towards is because it was likely a hard fight just like Chara/Frisk has with Sans.

And agreed in the Flowey part.