The criticism of events, both major and minor, just show people think it could be better.
Halloween is something the Simpsons has been known for. The THOH episodes are special to fans, and the early episodes used to come with tongue in cheek warnings from Marge with clever or genuinely 'scary' (okay, not actually scary.. but it had a darker or spooky element). So to see them using a Harry Potter parody from a weak later season is a miss for most who expected more - and know there is the content for a stronger event. Harry Potter is neither classic Simpsons nor at the peak of its popularity, so you just wonder why when there is a mine of material they could use and if it was a 'Magic' theme they were going for then why not another time as the Angelica Button stuff has been referenced many times and instead - as I don't like to criticise without offering alternatives - develop the Island of Dr Hibbert which could be far more than a single item sitting in the sea.
The Pagan stuff was a bit of a miss, too, because part of it was released before the episode it featured in was aired which was confusing at first and disappointing when that turned out to be it after previous Christmas events with presents and elves. I doubt the episode was memorable, either.