I do agree that there would be exceptions like mihawk and I also do agree that they might not have haki
But you are overlooking two key aspects here
First, all of them being weaker than orginals just take away the purpose of having them. Yes, complete subservient is big factor here but also strength. So, they are atleast on par with warlords.
Second, Boa df works only when her opponent has some emotions/feelings. Seraphims are emotionless so Boa would be having hard time