Never said the admirals are invulnerable. They're definitely far more competent than Big Mom though. Don't try to twist things and use this as some kind of excuse for Big Mom's incompetence. If Law and Kid used actual competence to outclass Big Mom, then we wouldn't be having a conversation about Big Mom's incompetence. Oda literally had to make a dumbass out of Big Mom to make Law and Kid pull off a win.
The Scabbards by themselves were puncturing holes in Kaido's body, but you never see me saying Kaido is incompetent because he is not, unlike his supposed equal. Arimaki was fighting multiple commander-level people at once when Yamato came in and attacked him. Yamato by herself is a far more competent fighter than Kid and Law, and she literally held off Kaido by herself for several minutes. So it's not an anti-feat for GB that he got hit by Yamato, who can tag Kaido in a one-on-one setting.