'over-breeding, crappy excuses and lazy owners' as you say, does in fact, produce the problems we see in rescues. Non-socialization, which is prominent in poorly bred, BYB bred animals, often = fear biting, runners, dog aggressiveness etc. Dogs left alone for months or years in back yards, often become diggers and/or barkers. This is what I consider 'baggage'.
My daughter's latest rescue, is a poorly bred Cardigan Corgie. Unusual dog to find in rescue, but nonetheless, came with many problems to be ironed out. Responsible owners though, can undo much of what rescues come with, if just committed enough.
I never wish to discourage anyone from rescue. We rescue horses and dogs and in the end, all have become quite model citizens.