I admit that I would not take on a "problem dog" either, Bernie. And I agree that it is more of a risk with larger breeds.
When I looked for another rescue dog, I was only willing to take on one that had had proper socialisation. And Banjo is fantastic with other dogs. And loves all people (a tad too much maybe ). It was unfortunate that no one taught her not to jump up on people when she was younger, but that is an issue I was willing to deal with. Aggression towards other dogs or people was not. Nor was an extreme desire to escape.
I am very happy that I chose to get an older rescue dog instead of a pup because that she had some of the traits I was after from what her foster carer told me.
But I have seen some cases at the RSPCA that I think sadly will never be rehomed because it would require a very committed person with lots of experience and time and energy to take them on.