He has always given plenty of effort but has in the past been guilty of losing the ball too often.
I have a theory, which might be barking up the wrong tree - but here we go:
He's potentially a (properly) brilliant player in a team that plays the game that Buendia wants to play. The quintessential Buendia move that I think is happening in his head, is a pass into his feet, which he knocks first time to someone eight yards away, he spins away expecting that player to pop it straight back to where he's moved to and he's scored, all in about three seconds.
The problem is, he's spent the last few years knocking it to Bailey / Watkins / Ings / Rogers / McGinn, all fine players in their own way, but nobody I'd expect to be the middle person in that three second manoeuvre. And I think a lot of the "he gives the ball away" stuff is because the current way things are played, we're all doing slow build-ups around the opposition box and drawing people towards you to create space - so very few players play first-time passes anymore and therefore aren't expecting a quick give-and-go to be happening. So when Buendia passes straight to Watkins and it bounces off him, Watkins is expecting him to take about six touches because that's what players do now - but "Buendia's lost the ball again".
I reckon that Buendia - Rashford - Asensio would have actually worked really well as they are the sort of players who would have played the game that Buendia wanted.
Obviously that doesn't mean we should be dropping Watkins and Rogers and building the team around him or anything. But there is probably a team out there with a very specific Buendia-sized hole where he could absolutely light up the league. For now though it's fine that he's doing what he's doing for us.
Or the concise version - he's a potentially brilliant one-touch player who has only ever played in a team of hold-the-ball-and-dribble-a-bit players and that will make him look worse than he is.
Edit - I also reckon this theory is given greater weight, by the fact that I think his most productive time for us is when he and Coutinho were bouncing off each other.