Dogs are in rescues for any number of reasons and many of them are perfectly lovely, affectionate and easy dogs who would fit happily into a household and bring nothing but joy.
But to say most are like that is naive.
The last time I volunteered at a local dog rescue there were about 40 dogs there. Approx. 5 had deep rooted behavioural problems as a result of deliberate abuse.
A few more had significant behavioural issues as a result of something happening to them outside their and their owner's control. In one particularly heartbreaking case a little old dog had a deep fear of men which had developed after her elderly owner became aggressive as a result of dementia. For ten years or more she had been a deeply loved pet and companion and that hideous disease robbed her owner of his mind and her of her home and trust in men.
Anything up to 25% had fairly significant behavioural issues as a result of lack of training, socialisation or proper care. In short, they had not have the upbringing you might hope for. This can be challenging. Loose lead training a very young labrador is one thing. Loose lead training 35kg of pure muscle is quite another.
Another 25% showed significant behavioural problems that may have been a result of being in kennels (it is a very stressful environment for many dogs) and so may not be an issue when they are in a home.
That left about 25% who were smashing dogs who would need no more than patience and care to settle into a new home and who would bring nothing but joy.
But (and it's a big but), in all cases you would know what you were getting into because the dog was already grown and developed. Puppies are much more of a gamble.
I've had 6 dogs in my life and 5 have been rescues of one form or another. Battendog (the latest after a looong gap and wait) was bought as a puppy. Anyone who thinks he was the easier option is fooling themselves. I love the little bugger like crazy but he has been, by far, the most hard work of any of the dogs.
Like some pp, I have no problem with anyone deciding a puppy is the best option for them so long as they have really and truly researched what they are doing and make a proper commitment to the dog. Those type of responsible people are no more to blame for the millions of dogs who need homes than someone who buys new shoes is for the amount of waste fly-tipped.
The people who ARE responsible for many of the dogs in rescue are the feckless bastards who allow indiscriminate breeding, who get a dog without any thought or commitment, who give up on the dog at the first sniff of inconvenience or who are so bloody terrible to the dog that someone has had to take it away. They are the shits who deserve the judgment and ire. May it rain down upon them.