I agree that they are astoundingly organized. I have wanted to implement some of their ideas in my house for a long time. My biggest issue with larger families is the amount of responsibility that falls on the older siblings with regard to the younger. I know, I know...that is what family is. But, my dh was the 2nd oldest of 7, and he feels that his parents took unfair advantage of him for free labor and babysitting. I know a lot of kids from big families who feel this way.
I'm sure it's different in every family. And the Duggar kids don't seem to mind. But, and mind you this is just me personally, I feel guilty that I have asked for so much help from my older kids with the younger. And, if I feel like this with 5, I know that I just could never do 18.
It works for the Duggars, and they make it
ALMOST 
look easy. They are amazing. I maintain though.....Better them than me!