If you're worried about stat's getting too high, and only want them to have a free feat and not an ASI, I would just say that to them.
If none of the book feats suit, ask reluctant players to work with you to make a custom feat. Even if they dont normally like feats, there will be something they'll like if they have a hand in making it. It could of course be a "half feat", with a +1 stat and moderate other bonus. If the player dislikes complexity, the moderate bonus could have a static quality, like bonus HP, adv on a skill or a more nebulous benefit left to the DM to determine, such as knowledge of local safe houses or streetwise contacts who feed him interesting/useful info from time to time.