It ain’t as bad as some reviews made it out to be, but I see where they are coming from—anyone without in-depth knowledge of Potterverse lore would feel like they are treading water. The show a lot of stuff that would be familiar to fans without any explanations, leaving the audience to connect the dots using existing knowledge of the franchise.
Also, in terms of writing and plot, expect Deathly Hallows Rowling and not Half-Blood Prince Rowling. If you, like me, disliked how arbitrary the Elder Wand ownership shenanigans in Deathly Hallows went down, you are going to hate the amount of shit Rowling pulled out of her ass in this one.
The final reveal of the film came SO out of nowhere that one can’t help but be shocked, but it would be a mistake to consider it good writing. When the film ended, you’ll have more questions than answers. “RUH?” was how I felt leaving the theatre.
The Crimes of Grindelwald is a table-setting movie in that it resolves nothing but exists solely to put things in place for the next three entries in this film series. And if one considers this series to be a TV show, it would have been considered a solid wham episode but as a film, it cannot stand on its own.
And to put a controversial opinion out there: Depp is a lot better here than he was in the 1st Fantastic Beast film, and proved to be a far more interesting and charismatic villain than Voldemort ever was.
Also I think as far as prequel films go, the Fantastic Beast movies (both of them), proved themselves to not be unnecessary. Some prequels retroactively ruin the properties they are based on, like that Solo film. The adventures of Newt Scamander and hot daddy Dumbledore expands on the wizarding world in very interesting ways—though in some aspects, it also made it smaller (like that shock ending).
Go see it if you are a Potterhead. There is much to love and enjoy, and if you aren’t one, bring a Potterhead with you to explain things to you after the credits roll. That Potterhead friend of yours should ideally be well-versed in Dumbledore and Grindelwald lore that goes beyond the original 7 Potter books, and be aware of the canon titbits that Rowling been dropping into the fandom via social media over the years like rat droppings.