Doing a prequel was always going to be dicey, but had they taken a lot of care with the material, it might have worked out. Sadly, not much care has been taken. The first big controversy related to to casting for Nagini. To many, it seemed like a desperate ploy to introduce more diversity in the universe. But perhaps Nagini was the wrong choice; as a character who eventually becomes enslaved by Voldemort, a white man, it gives a very horrifying read. Then there's the fact that "Nagini" is, at its origin, a Sanskrit word that comes from India. So, while it was admirable to cast a Korean woman at all, it seemed strange to make the choice to cast her as Nagini.
It's not just off-key casting choices that have unsettled us, either. There seems to be a quite blatant disregard for Harry Potter canon in other ways. The sorest thumb is the appearance of McGonagall, who, according to the books, was born in 1935. This movie takes place in 1927, eight years before her birth, and yet she appears as a 20-something professor at Hogwarts. I guess you could claim she used a time turner or whatever, but that sounds more like convenient plothole repair than an actual explanation.