Three of my five current dogs are from shelters, two I got from a friend. I like shelter dogs, obviously, but there are a handful of breeds I personally would not get from shelter unless they were puppies. Pitt bulls, Dobermans Rottweilers any of the guarding shepherd breeds (i.e German, Belgian etc) to name a few. Basically I exclude most of the working/guarding type breeds from what I would adopt from a shelter unless it was still a pup. Let me explain why.
The above breeds are some of my favorite breeds, and if I could find one as a puppy at a shelter I would have no problem adopting it. But to adopt one as an adult without knowing what its previous situation, training, and socialization has been I feel would be too risky. Years ago we acquired an adult German Shepherd who'd been abandoned. While she was loving at home with family she turned out to be a lawsuit just waiting to happen. Very human aggressive with strangers and clearly under socialized.
Pitt bulls used to be a breed I would not have considered getting from anyone but a breeder because of the difference between how that dog is bred on the street vs. legit breeders, specifically how "street" pitts are bred to be more aggressive in some cases, however after taking in two pitt mix pups (possibly with rottie) I realized that even dogs from lines that may be bred to be more aggressive, if properly socialized and trained, can make excellent pets.
So to sum up my answer, I adopt my dogs as adults or puppies as long as they are not one of the above listed breeds. The above listed breeds I would only adopt if they were under a year old, otherwise I would get them from a breeder.