Actually it's hard to give a definitive opinion on the writing, when you read it mostly translated - so much depends on the translator. I read The Plant and Gwendy's Button Box in English recently. In the first I had to look a LOT of words up - plus there are numerous references to American personalities/events etc. that are unknown to me; I don't know if they normally take these out of translation, but I assume they do, as in the translations I never come across that many unknown personalities.
Gwendy wasn't hard to read.
But it isn't so much the writing itself - that never changed so much as far as I can tell, it's just that the earlier, more purely horrorstuff grips me more and is more fascinating to me. Those earlier books seem more focused, they don't go in so many different directions than some of the later ones. Again, you can't say it as a rule, but broadly speaking.