Some general observations in relation to the OP
@Fortitude
- some players are 'uncoachable' in the sense that they are mavericks, blow hot and cold and cannot be trusted to follow/carry out instructions. These players obviously should not be considered top players in the modern era. I'm thinking here the likes of Allan Saint-Maxim, by way of example.
- some players are 'uncoachable' in the sense that they are one-dimensional and/or simply cannot play the way a given manager wants them to play. I'd use Rashford as an example here. He has raw attributes but he's never progressed as a footballer beyond a wide forward who can cut-in and shoot from the left-flanks. Various managers have tried and failed to make him effective on the right, as a CF or to coach him to press better etc...but he's really added nothing to his game since he first burst onto the scene as a teenager.
- some players are 'uncoachable' because they are too old and/or too used to a completely different style of football. For example, I question the underlying wisdom of ETH trying to coach new ideas into the likes of Evans, Maguire, Varane, Casemiro etc...at this stage in their careers.
See the Evans interview after the Burnley game, in which he stated something along the lines of 'its been difficult to learn new things at my age, being asked to pass the ball and run forward is completely unfamiliar to me as a CB!".
LvG was famously on-record many times as saying he much preferred to coach younger players, as they were more adaptable, eager to learn and hadn't picked up 'bad' habits.
- I note Bruno has been made this seasons scapegoat by the fans, but all I see if a player doing everything he can to do exactly what his manager is asking him to do. He's pressing, he's playing wide and he's getting the ball forward as quickly as possible. I think it's difficult to be critical of his ball retention etc...when he's clearly following instructions. So with regard 'is he coachable?' I'd say he is...but the coaching/instructions he's receiving aren't helping him.