there are have been accusations that her work is very derivative and I've been told that her use of English is somewhat stilted and facile
Two things to unpick here. She's writing for children, yet people seem to want to compare her to a Booker Prize winner. I think her work is very readable, she draws you into her world and I think her dialogue is good. However, there is a snotty feeling among "serious" writers that you shouldn't use adverbs – ever – and JKR does, a lot. eg "Hermione said sharply", "Harry said brightly". I can't not hear/see that, now that I'm aware of it. I can see why people are critical of that.
Second thing is this "derivative" comment, which is always levelled at JKR. I don't think it stands, myself, because although other fantasy writers have created whole worlds/creatures/magical powers from nowhere, her whole schtick is that "we" muggles are, on a small level, aware of the magical world, we just don't believe in it (because we are prevented from knowing for sure). So yes, she didn't create vampires or werewolves, and her spells are derived from classical language, and her magical plants are familiar to us, but I think that's because it's meant to be "real"; she's weaving that mythology into a familiar-feeling world that includes King's Cross and West Ham FC and people wearing jeans. I think that's why it appeals so much to children, because it's "real" and "not real". (As with so many famous children's fantasies – The Lion, the Witch and the Wardrobe, The Borrowers, Northern Lights.)
If it's not clear, I think J.K. Rowling is fantastic and I condemn the vile way she has been treated of late.