I feel for you. You don't know what their life was like, so you don't always know what they will react to.
My Hunter was about 1 year old when we got him. He had been wandering the streets for a few days at best guess. No idea exactly how old. Integrating a dog into a highly active lifestyle was difficult at times. We were trail running, hiking, camping, and off on multiple day rafting trips. So quite a change for a dog that seemed to be completely out of shape. He would get tired just walking around the block. Can you imagine taking a dog rafting that had never seen water or been swimming before. He was never terrified, but he also wasn't super excited to go on our trips.
His biggest problem was his complete lack of socialization as a pup. He was and still is dog aggressive. Any large male or female dog he puts his hackles up and wants to push up against them. I have to watch him closely. He's gotten so much better with a lot of work.
I think with any rescue that lack of socialization is pretty common.
It would be interesting to see what rescue groups have to say about what they see.
~ Stacey with Hunter, Lucy and Riot, missing Reilly, Tiger and Pennie