i somewhat agree... there is no magic wand or silver bullet or guarantee... however, the behaviorist is still much more informed than the vet for this.
more importantly, there are behaviors that can be predicted from a very young age. skittishness/fearfullness is a great example. it is very very hard to train this out of a dog. as we all know in dogs, its either fight or flight. a skittish dog will choose flight most of the time. when it cant flee, it will have to fight. this will lead to dog bites, especially with children (specifically under 10yrs old) and you don't want that. there are tests you can do with a puppy that will illustrate this behavior. a simple one is how it reacts to a loud startling sound like dripping a clip board. you will want a dog that runs to the clip board to investigate or one that does not react at all. one that cowers and runs from the noise? don't adpot unless you want to do a lot of work.
i also agree that breed characteristics are something to at least pay attention to... beagles = hound dog (if I am not mistaken) = tend to be loud/alterers... howlers, barkers. also tend to be fairly independent. they tend to be very loyal and family oriented (they hunt in packs). tend to be very smart and easy to train but supposedly require a firm, consistent hand.
again, all of this is general knowledge... there are uniques and exceptions and of course each dog has its own personality which is why we love them so much.