“Realistically how well do the rescue centres know the dogs if they haven't been there that long.”
They do full behavioural assessments - it’s not just a case of it being the opinion of whoever has spent some time with it.
“Something may trigger a negative memory in them and they may snap”
Dogs don’t just snap, not really, so yes it’s impossible to predict how a dog will react in every circumstance... but, they will have assessed them and know if they’re nervous, likely to get stressed by common things or pretty well adjusted.
Most rescue dogs haven’t had a horrific start to life, by far the biggest group of dogs to end up in rescues are teenagers who have been perfectly well looked after, just nobody really thought about how they were going to train or exercise the now adult dog when they got a puppy and then gave it up when it was bored and misbehaving. All they need is exactly the same basic training you’d give a puppy.
You’ve also got dogs who have been much loved pets, but things like divorce or bereavement have meant someone is unable to keep them.
It really doesn’t happen often that dogs have major behavioural issues that are only discovered when they’ve been rehomed, rescues always err on the side of caution if an adopter has children, that’s why you see so many dogs advertised in rescues that are unsuitable for children or children under a certain age, more often than not it’s becayss they’re a bit bouncy or they’re large and clumsy rather than because they’re likely to bite, dogs that are likely to bite are advertised as requiring specialist homes and the input of a behaviourist.