I got a rescue from the pound a couple of years ago, Lab/Hound mix. He was just over a year old. He was picked up as a stray with his brother. He had a skin disorder where he had all these hairless spots all over and no hair on his belly and chest. I was told it was hot spots but turned out to be allergies. I changed his diet and the condition cleared up.
Anyway, he was always real skittish of everyone at first and it took a couple of months to warm up to me. He finally warmed up to the rest of the family within the year. He is an indoor/outdoor dog (comes in at night and bad weather). He has good temperment, I have never seen him get aggressive with strangers or other dogs. The thing is he is still very skittish when someone comes around that he doesn't know. He is okay once he gets to know you but it takes a while (2-3 visits with treats) for him to build up any trust. Like I said he is not aggressive, he just barks and runs away keeping his distance.
My thing is I have had him a little over 3 years now and although he does seem like he has warmed up to things he still has some apprehension at times, even with me. I tell him all the time (yes, I do talk to the dog) that he is not 100%. More like 85-87%. I feed him every day, let sleep in the house every night, and have never done anything bad to him but yet every now and then he will get in a mood where won't come to you when you call. he will keep his distance like he thinks you are going to do something to him. I tell him he has "real issues". But even with all that he is a good dog and I still like him. He is just a little weird.
Anyway to sum up all that. I have 4 dogs. A cocker spaniel, 2 bird dogs, and the lab/houd mix. I raised the first 3 as small puppies and got them off their mothers. They are all 100% my dogs. Everything they do is for me and come whenever I call. Loyal to the end. The lab mix on the other hand seems pretty loyal most of the time but gets crazy sometime out of nowhere. He has never been abused or mistreated for all I know. Lives a pretty good life but now and then he will act like he doesn't trust you. Then on his own terms he will come to you when he wants to. But on another note I have gotten a "true rescue " before where I am pretty sure she was mistreated and she turned out to be one of my most loyal and sweetest dogs I ever had. So like you said. I think it is a crapshoot with rescues. Also for what it is worth I will probably get another rescue when this one is gone.