From a metal fan's perspective on their music (I was unaware of the problematic statements until today that
@Teeeee mentioned above, so maybe there are other reasons people hate them too):
There are a few camps on this.
1. Metalheads who HATE them because they can't stand any scent of core or post elements in their music. They are close-minded and metal elitists.
2. Metalheads who like their stuff on its own merit and make unbiased judgments based on how the music makes them feel regardless of any genre restrictions.
3. Metalheads who are aggravated by the press coverage Deafheaven got because they have some posty stuff in the music so people actually listened to a full metal album for a change and crowned them as saviors of a dying genre (totally false and irritating). They don't actually hate Deafheaven, but resent the undue attention heaped on them when what they are doing isn't all that fancy or worthy of the hyperbole of being a savior of black metal. I happen to fall into this camp.