I have also read them aloud to my DS and found them difficult in the mouth, though I think it's the combination of prose that is serviceable at best and the increasingly endless detail as the series moves towards its close, that makes it unpleasant to read aloud, and I always find myself wanting to cut as I go.
Enid Blyton is equally banal as a prose writer, but has a lighter touch as a world-builder, and of course wrote these short, strictly-disciplined, formulaic novels, and wrote them very rapidly. She would knock out a Famous Five novel in a week, rather than the increasingly length, complexity etc of the HP novels.
I think the HP series just changes so much as it continues, and part of the issue is JKR's disabling sense of responsibility to her own massive fan base, who just wanted more words, more detail, more backstory, more explanation of how magic concept x actually worked, so every possible pudding got over-egged, stopped working as novels, plus there was the problem of who exactly the implied reader was -- a child? A young adult? Someone who had grown up along with the series? It's mirrored in the twee Disneyish Chris Columbus early films, and the much darker tone that Alfonso Cuarón introduced.
Have also read some of them in French to my French godchildren, and wasn't keen on the French translations, either. The Irish version of Philosopher's Stone was better, I thought. I do feel a lot of affection for the series, though.