I don't really think that's the same thing, tbh. Her employer was the prince of the empire, and she's the leader of an assassination guild hired by the prince to kill the princess of a neighboring kingdom, and who would've succeeded if not for MC creating a curse nobody would think should be able to exist (basically rivaling the demon king's heart but with no bad effects). Not going against a prince throwing a tantrum isn't weakness, it's just not wanting to piss off the royal family, and there's no reason to really doubt the abilities they're supposed to have in-universe.
I won't deny they come across as also stupid, but I think there's a difference between "they should be written stupid because they're oafs like Kash's group" and "they were portrayed as supposed to be better than this, so it comes off as dumb when they have such obvious blind spots". But I guess that's just the kinda manga this is.