My sister has adopted a lot of previously abused dogs. The best german shepherd in the world (according to her vet) was a previously abused dog. But the dog was also kinda neurotic. I dog sat for a long weekend, and she wouldn’t eat the whole time I had her. I microwaved the food, added broth, all the stuff my sister recommended. (And it was her regular food, that my sister delivered with the dog.) She was just too anxious away from her people, and probably afraid bad things would happen to her again.
All of her abused dogs have had issues. I mean, they’ve also been great dogs, but it’s a lot more work than starting with a puppy. I suppose a dog that’s surrendered because its owner has to move to a nursing home or something is probably easier than an abused dog. But they tend to still pine for their beloved first master.
The other problem with older dogs is that they die so soon. My sister is grieving her dog an awfully large fraction of the time. She takes fabulous care of them, but dogs just don’t live all that long.