I think the reason why so many people don't like it is because of all the background info required to understand this movie. This poses a problem due to the fact that Fantastic Beasts 1 came out 2 years ago and the original HP series came out even longer ago. I do agree though that there are a few problems with canon like Professor McGonagall appearing in a flash-back when she shouldn't have been born yet and other bits of fan-service such as mentioning McLaggen and how Gridelwald saved his follower at the start of the movie for no reason. My last complaint is a bit larger and is about Nagini and Leta Lestrange. Nagini basically did nothing except for look concerned for Credence and her circus performance didn't make any sense since Animaguses have been shown to be normal by how nobody flinched when Mcgonagall turned into a cat in Philosophers stone. My problem with Leta is that she wasn't given any time to be developed and we didn't know very much about her resulting in me having no sympathy when she revealed the truth about Credence and no sadness when she died at the hands of Grindelwald
Apart from this, I enjoyed the movie very much and thought it was very good.