Don't really get the negativity and criticism he gets.
I am negative about him, from the same reasons why I don't "like" Alexis, Deulofeu and him (Bartra).
Alexis, he was a very good player, but posts by his "followers" how he delivers sex, how he is almost the greatest player who has ever walked on Earth, start to create negative feelings about the player, at least for me (and I guess for some other also).
If Alexis wasn't hyped that much from his fans, nobody would care, and I would always stay "neutral" about him.
He is neither awesome, and neither a total crap.
Deulofeu, also. The hype about him is insane, even though he hasn't done anything at a senior level yet. He is a huge flop for now (compared to what he could have developed into).
And posts how we should get rid of xx, yy, because Deulofeu needs to get his chance at Barca, makes you think: "Wtf? What he has done to get a chance and that we should kick some proven players because of him?"
The same is with Bartra.
Whenever a squad before a match is announced, majority of our fans post: "Oh my god, I want Bartra on the field."
Ok, but why not Pique? We won so many trophies with him. Why nobody mentions Pique, people just want Bartra (because he is a new guy, a fresh story, more interesting story, I guess). Nobody cries when Pique is benched.
Vermaelen? Not too many fans will write: "Omg, I want Vermaelen in the team." And not too many will cry if he won't play.
So, basically posts how Bartra is a new Puyol (which he'll never be), a new Baresi (Bartresi) and similar exaggerations create negative feelings about players who are too hyped and who are not as good as hype about them claims.
With Bartra:
1. if he plays well, people ask for him to be a starter all the time
2. if he doesn't play well, people will say that he was rusty and how he needs more regular playing time
3. if he makes a rookie mistake, people claim how all defenders make mistakes, so we can't blame Bartra either
4. if he makes a huge mistake, like against Bale, then people claim how he was injured and similar
Basically, if he does something good, he will be hailed for days.
And if he does something bad, people who love him will always find excuses.
So, lack of any objectivity and criticism and too much hyping, sometimes create negative feelings in some other fans.
Alexis and his sex delivery, Deulo as a new Messi/Ronaldo, Bartra as a new Puyol.
Disclaimer: I am a Barca fan, not a hater, but I can't buy every single story and hyping how all of our new/young players are a future goats on their positions.