The monsters are great in this movie - the people, not so much...
The basic story of this movie is simple: Monsters are battling for dominance. But it is amazing how *poorly* that story is told. There are three major problems: Acting, pacing, explosions.
King of the Monsters is full of A-list actors who are doing a bizarelly uninspiring job, seemingly phoning it in (The monsters have more of a personality than the main protagonists). As a result, it's difficult to care about any of the characters in the movie. Two major characters sacrifice themselves, but you won't feel anything other than mild confusion. The main villain is such a ridiculous "bad guy" trope that you'll have to laugh every time he appears on screen (Which, thankfully, he does not do too often.)
The pacing of the movie feels odd. Everything is happening super fast, and the number of cuts per minute is off the charts. The movie hardly ever takes its time to explore either its characters' motives or some parts of its world. For example, there are dozens of interesting things about the Godzilla movie universe that are only shown or teased briefly (often literally for less than a second) and then ignored thereafter, because the movie is immediately jumping to other things...
The explosions, oh the explosions... Sure, in a Godzilla movie, there's going to be explosions! But the human-centered parts are so full of gunfire, explosions, yelling, and nauseating shaky-cam that you'd be forgiven to simply walk out of the movie - the sheer dreariness and boredom-inducing monotony of it all is agonizing. I can't imagine what they were thinking. It's just so throroughly unpleasant to watch. And you soon learn that all of this "action" is irrelevant, anyway: The main characters survive absurd deathtrap after absurd deathtrap.
The parts of the movie that feature the monsters are great, though (especially the end). If you find yourself watching this movie at home, feel free to skip through the human scenes and focus on the monster parts.