Admirals know something called dodging and Observations Haki. Bm was not familiar with this concept
I agree
Big Mom's greatest weakness is her mind and her overreliance of her natural abilities.
She's always been strong and durable as a kid and was basically never set straight by any adult. This has continued literally all of her life as she's basically a grown child at this point. Since she's too used to shrugging off attacks it's easy to catch off guard with an attack that actually hurts her.
And the grown child part affects her thinking as well, she has some clever moments here and there but in pretty much any scenario she's been in she lost due to being outsmarted by her opponents.
That's why I personally think why the admirals are so scary. Because not only are they insanely strong but they are smart and disciplined individuals as well. You don't need to have iron skin when you have the aptitude to dodge or counter attacks.