If the breeder is a reputable and good breeder, the pups will not learn fears as someone mentioned, and they will develop good habits while at the breeders...
Obviously, if it's a breeder who doesn't put in the time, at 12 weeks, you might get a dog who has developed bad habits and fears, but that's why you should buy from reputable breeders that *care* about their dogs and want them to be socialized and start the potty training process before they go to their new homes.
Also, what about bile acid testing?? 6 and 7 weeks is so early...I believe Dr. Center (the vet who invented the bile acid test and has been heavily researching liver shunts/mvd) recommends that they be at least 9 weeks, but prefers that they be 16 weeks...
So many health issues can come up when they're so little....They are not even supposed to have their first shots at this point!! Parvo kills....
