Karamoko is an interesting one. Guy I know does physio work with the Celtic youth sides and a few years ago all he would go on about was this kid, at a very young age they genuinely thought he was one of the biggest talents they'd seen, really special. Think size and injuries contributed to it not working out, but possibly also the player expected too much too soon having played age groups up most of his youth career.
The fact he didn't get anywhere at Brest is a concern, but he's only just turned 22 and if he's matured both physically and, more importantly, mentally the word has always been there's enough talent in there to be a top player.
I have connections at Celtic mate and i lived in Glasgow for a bit, the first time i heard about him he was 10 and you are right, they were beating PL clubs of with a stick back then, not sure what happened but they thought they had something really special. Im sure there is old video somewhere with him destroying teams of a higher age.