For what it's worth (and I'm not setting us up to be some paragons of virtue here), we've taken on a number of dogs over the last 20 years who'd be considered unsafe or unadoptable.
One was even in his last two days before being PTS at the shelter at six months old - he had warnings all over his paperwork and we were told there was no way he'd ever be safe, especially with children. The only time he ever came to the front of his enclosure instead of hiding at the back and baring his teeth, though, was when our daughter went to look at him. Nearly bent himself in two wagging at her. We had to sign a waiver, but there was no way we were going home without him. Damn dog hated me for 15 years (until he had dementia and forgot he hated me every morning), but only had two minor bite incidents - and those were people who ignored all of our warnings and shoved their hands in his face.
Then there's the Akita, who was unadoptable because of her size and "scary" posture. Most well-behaved dog we've ever encountered. She basically raised the feral Chihuahua (who's now the most placid example of the breed we've come across), and the traumatised Romanian rescue (who would apparently never accept human contact, but now can't live without fuss and sitting with us at every opportunity).
Point is...we aren't miracle workers, and I know there are many dogs out there that we wouldn't be able to rehabilitate, but for an awful lot of them it's because the environment we provide isn't what they need. I'm convinced that many "unadoptable" dogs just need to find the environment or people that suit them most, and they suddenly become unrecognisable from the scared, fear-aggressive selves they showed everyone else (and us, to begin with) - for the dogs we've had (all of them), it's been down to treating them as individuals instead of seeing teeth or a bite incident and thinking "Yep, aggressive" and writing them off - by rescues, fosters or adopters.
I don't think that applies to the OP's experience, though, because it was doomed to failure the instant the rescue decided to hold back the dog's history. That's just bloody stupid and irresponsible.