There is a reason why, for example, music artists have a producer - it's hard to notice one's own faults, and the producer can to a much higher level see what can be improved and steer the project in the right direction. The same goes for films of course, and here is a film that is written and produced by the same person - J.K. Rowling. And my guess is that no one dared challenge her on this idea and how it should be written.
The main problem is that the script is bad. It lacks character development. It lacks humour. It lacks flow. It lacks sharpness. It is confusing most of the time. On numerous occasions I found myself not understanding what I was watching. That is either explained by a very bad storytelling, or a sudden onset of dementia on my part. Who are these characters? What do they want? Why are they there? Why is Graves involved with some sort of orphan boy, looking for a special child? Who was this child supposed to be? I never found out! The idea is good, it's just not handled well. Rowling could have skipped that dark part of the story and just concentrated on the fantastic creatures and the fun things they would do. It lacks OUMPH as it is now.
Another problem are the actors. Stiff, stale-faced people with no quirks or interesting personas. The only one I found interesting was Alison Sudol portraying Queenie. Great energy and charisma! She could go far. The rest, including the more famous actors, acted poorly to say the least.
Sharpen up next time! (Hire me)