I took on a pair of cats who had spent their first year as part of a feral colony, though they had spent the next year at the rescue failing to be rehomed. They mainly didn't go because a) they were black, b) bonded pair who had to go together and c) incredibly shy & would hide away from everyone.
Also, I had always wanted indoor only cats and this was a condition of their adoption (either that, or farm semi-feral). The reason for this is that even if kept in longer than most do after rehoming, they could go feral again very easily especially if they get frightened.
Brought them home & kept them in the living room at first, didn't even see them for days (behind the sofa) though food and water were going overnight and the tray being used. Took me weeks before I was letting them in other rooms, 6 months before I got to stroke Girlcat and even longer for Boycat. They didn't make a sound for the first couple of years.
Fast forward 5 years, and they are the most demanding cuddle monsters. Girlcat squeaks at me very loudly if I'm not paying attention to her, Boycat makes little sort of growly murmurs when he is summoning me to the sofa for belly rubs. They will both occasionally let out a cry for attention too.
I am absolutely besotted with them still, and would definitely call mine a very positive story. My number 1 tip would be patience, patience and more patience. I let them come to me in their own time, just made sure I spent most of my time in "their" room and talked to them conversationally e.g. commenting on the TV programmes I was watching. I made sure we had a routine e.g. at bed time I would do everything in the same order, talking to them, and always leaving a few dreamies out. Nowadays I get pestered with quiet yowls from Girlcat if I'm late up to bed!
Good luck, and don't forget to pay the Litter Tray tax when he comes out of hiding 